Config based routing

[marcin-krolik]

Fixes #539

Summary of changes:

• removed SelectablePlugin interface, since AvailablePlugin interface can be used as well. SelectablePlugin did not introduce any usable abstraction in my opinion.
• aliased map[uint32]AvailablePlugin to MapAvailablePlugin to ease working with keys/values of map
• implemented naive config_based routing based on sticky routing implementation

@intelsdi-x/snap-maintainers

[IRCody]

What was the reason to remove this?

[marcin-krolik]

This change should not be removed. Thanks for noticing!

[jcooklin]

Do you want to override the currency count here or allow it to be defined by the plugin author?

[marcin-krolik]

Good point. I forgot this value can be provided via plugin metadata.

There's one more thing regarding concurrencyCount. I think I found an error in pool.Eligible() that for p.concurrencyCount != 1 function pool.Eligibility() does not work properly.

Main condition:

should := p.SubscriptionCount() / p.concurrencyCount
if should > p.Count() && should <= p.max {
    return true
}
return false

p.SubscriptionCount() and p.concurrencyCount are both int

Let's say p.Count() = 1, p.max = 3, p.SubscriptionCount() = 4 and p.concurrencyCount = 3 then should = 1

Main condition is false, thus pool.Eligible() = false while in my opinion it should be true

Maybe it should be reimplemented to work on floats?

[marcin-krolik]

Tests fail because of #910

[pittma]

// config-based provides a strategy that selects a plugin based on the given config

[pittma]

Dude, Marcin, I love this! It looks so good. Would you mind adding a test to pool_test.go around SelectAP?

[marcin-krolik]

@danielscottt : Thanks! I wrote some additional tests for pool.SelectAP(). take a look and say what you think!

[pittma]

LGTM!