New accounts are 'pushed' to worker threads

[vtnerd]

If a new account is added via cli or REST api, the status checking thread will "push" (via ZMQ) the new accounts to 1 of the scanning threads (chosen via fair queue). The current behavior is to reset all of the scanning threads. This should have better performance when lots of accounts are being added to a server.

I decided to use ZeroMQ instead of a lock with c++ queue so that this could be expanded into pushing accounts into other processes entirely. That effort is a difficult one for sure, but worthwhile if it can be achieved (I am not certain whether ZMQ can be used for that - or some other solution).

The account -> msgpack -> account step should be fairly quick, given that integer keys are used, etc.

[vtnerd]

Force pushed a rebase after some merging onto develop branch.

[vtnerd]

The account -> msgpack -> account step should be fairly quick, given that integer keys are used, etc.

I profiled the time to serialize an account with a testnet account with many transfers (mostly spends). It took ~280 microseconds to serialize and ~500 microseconds to de-serialize. So transferring accounts via ZMQ is indeed fairly quick.

With that said, this may change to a c++ queue with lock because ZMQ across processes just isn't going to work.

[vtnerd]

Force pushed another rebase.

[vtnerd]

I might change how this works slightly, as I think the worker threads should immediately register for PULL socket instead of waiting until middle of the scan loop.

[vtnerd]

Rebased again after recent merges.