Support wildcard subscriptions + getter for known types (optimized)

[raulk]

This PR supersedes #34.
Implements libp2p/go-libp2p-core#153.

It implements wildcard subscriptions differently, by adding a dedicated wildcard node. It also validates emitters and subscriptions:

• Rejects Emitters for wildcard subscriptions.
• Rejects variadic subscriptions that include the wildcard. (Wildcard subscriptions must be started separately)

Benchmarks against master

Without atomic wildcard sink counter

benchmark                                               old ns/op     new ns/op     delta
BenchmarkEvents/subs-001/emits-001/stateful-false-8     124           146           +17.74%
BenchmarkEvents/subs-001/emits-016/stateful-false-8     191           206           +7.85%
BenchmarkEvents/subs-001/emits-256/stateful-false-8     203           230           +13.30%
BenchmarkEvents/subs-016/emits-001/stateful-false-8     3253          3299          +1.41%
BenchmarkEvents/subs-016/emits-016/stateful-false-8     3385          3028          -10.55%
BenchmarkEvents/subs-016/emits-256/stateful-false-8     3513          3611          +2.79%
BenchmarkEvents/subs-256/emits-001/stateful-false-8     50084         49766         -0.63%
BenchmarkEvents/subs-256/emits-016/stateful-false-8     47577         45560         -4.24%
BenchmarkEvents/subs-256/emits-256/stateful-false-8     44206         47754         +8.03%
BenchmarkEvents/subs-001/emits-001/stateful-true-8      120           148           +23.33%
BenchmarkEvents/subs-001/emits-016/stateful-true-8      200           207           +3.50%
BenchmarkEvents/subs-001/emits-256/stateful-true-8      214           229           +7.01%
BenchmarkEvents/subs-016/emits-001/stateful-true-8      3407          3218          -5.55%
BenchmarkEvents/subs-016/emits-016/stateful-true-8      3308          3503          +5.89%
BenchmarkEvents/subs-016/emits-256/stateful-true-8      3627          3775          +4.08%
BenchmarkEvents/subs-256/emits-001/stateful-true-8      50086         47147         -5.87%
BenchmarkEvents/subs-256/emits-016/stateful-true-8      46958         51912         +10.55%
BenchmarkEvents/subs-256/emits-256/stateful-true-8      45118         46186         +2.37%

With atomic wildcard sink counter (this PR)

⟩ benchcmp {before,after2}.txt
benchmark                                               old ns/op     new ns/op     delta
BenchmarkEvents/subs-001/emits-001/stateful-false-8     124           125           +0.81%
BenchmarkEvents/subs-001/emits-016/stateful-false-8     191           178           -6.81%
BenchmarkEvents/subs-001/emits-256/stateful-false-8     203           209           +2.96%
BenchmarkEvents/subs-016/emits-001/stateful-false-8     3253          3236          -0.52%
BenchmarkEvents/subs-016/emits-016/stateful-false-8     3385          3337          -1.42%
BenchmarkEvents/subs-016/emits-256/stateful-false-8     3513          3651          +3.93%
BenchmarkEvents/subs-256/emits-001/stateful-false-8     50084         46929         -6.30%
BenchmarkEvents/subs-256/emits-016/stateful-false-8     47577         52067         +9.44%
BenchmarkEvents/subs-256/emits-256/stateful-false-8     44206         44952         +1.69%
BenchmarkEvents/subs-001/emits-001/stateful-true-8      120           128           +6.67%
BenchmarkEvents/subs-001/emits-016/stateful-true-8      200           181           -9.50%
BenchmarkEvents/subs-001/emits-256/stateful-true-8      214           201           -6.07%
BenchmarkEvents/subs-016/emits-001/stateful-true-8      3407          3361          -1.35%
BenchmarkEvents/subs-016/emits-016/stateful-true-8      3308          3530          +6.71%
BenchmarkEvents/subs-016/emits-256/stateful-true-8      3627          3595          -0.88%
BenchmarkEvents/subs-256/emits-001/stateful-true-8      50086         48324         -3.52%
BenchmarkEvents/subs-256/emits-016/stateful-true-8      46958         51369         +9.39%
BenchmarkEvents/subs-256/emits-256/stateful-true-8      45118         52283         +15.88%

[aarshkshah1992]

This is beautiful.

[raulk]

Gonna merge because I need to make progress. Feel free to review after the fact.