Simplify use of SendTransactionService

[garious]

Problem

Our services typically communicate with channels. By using channels, we can easily swap out what's on the other end. In the case of something upstream from SendTransactionService, it's sometimes useful to swap out SendTransactionService for something that doesn't require a UDP socket.

Summary of Changes

Refactor SendTransactionService to send transactions after receiving them instead of just before sending. This allows us to remove the send() method and expose the Receiver<TransactionInfo> directly. Upstream users of the service then no longer need to hold the object unless they intend to call join() on it. Instead, pass the upstream a Sender<TransactionInfo>, which may or may not have a SendTransactionService on the other end.

[codecov]

Codecov Report

Merging #10999 into master will increase coverage by 0.0%.
The diff coverage is 91.8%.

@@           Coverage Diff           @@
##           master   #10999   +/-   ##
=======================================
  Coverage    82.2%    82.2%           
=======================================
  Files         318      318           
  Lines       72870    72866    -4     
=======================================
+ Hits        59914    59931   +17     
+ Misses      12956    12935   -21