[1.0-beta4] Some misc improvements

[heifner]

• Add missing mutex to fork_database_t::has_root().
• When a block fails to produce, do not attempt to produce until the next block time stamp. This prevents a tight spin on failures. We had an issue somewhere for this, but can't find it. I found the problem when testing a possible alternative to [1.0-beta4] Misc improvements #448.
• Refactors terminate-at-block handling into a function.
• Additional std::move in assemble block.
• Logging improvements
  • Do not log expired transaction count during replay.
  • Do not log expired transaction count if there are none.
  • Remove confusing "maybe switch forks" log statement.
  • Log block timestamp instead of wall clock time (which is always logged anyway) during start block.

I could separate some/all of these into different PRs if desired.

[linh2931]

I like this removal. Was thinking about removing this spamming removed 0 expired transactions... too.

[ericpassmore]

Note:start
group: CLEANCODE
category: INTERNALS
summary: Improvements to Finalizer, ForkDB, and Controller. Refactors code for readability, adds logging, and improves concurrency with additional mutex.
Note:end