Use subscriptions to detect tx inclusion when sending txs

[sisou]

What's in this pull request?

Change transaction-awaiting in client.sendTransaction from an inefficient polling mechanism to re-use the client's address-event subscriptions introduced in #1437.

Also includes a small performance optimization in client.getTransactionsByAddress that returns early when there are no transactions to get.

This relates to #1339.

Pull request checklist

• All tests pass. The project builds and runs.
• I have resolved any merge conflicts.
• I have resolved all clippy and rustfmt warnings.

[codecov]

Codecov Report

Patch coverage: 5.26% and project coverage change: -0.18 ⚠️

Comparison is base (913c1e0) 64.60% compared to head (b8a6069) 64.43%.

Additional details and impacted files

@@              Coverage Diff              @@
##           albatross    #1509      +/-   ##
=============================================
- Coverage      64.60%   64.43%   -0.18%     
=============================================
  Files            426      426              
  Lines          52951    52983      +32     
=============================================
- Hits           34208    34138      -70     
- Misses         18743    18845     +102     

Flag	Coverage Δ
unittests	64.43% <5.26%> (-0.18%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
web-client/src/lib.rs	0.12% <0.00%> (-0.01%)	⬇️
web-client/src/transaction.rs	0.00% <0.00%> (ø)
consensus/src/consensus/consensus_proxy.rs	43.60% <80.00%> (+0.31%)	⬆️

... and 10 files with indirect coverage changes

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