3.x porting - add remonitor code to DOWN message (#3144)

[jiangphcn]

Overview

Smoosh monitors the compactor pid to determine when the compaction jobs
finishes, and uses this for its idea of concurrency. However, this isn't
accurate in the case where the compaction job has to re-spawn to catch up on
intervening changes since the same logical compaction job continues with
another pid and smoosh is not aware. In such cases, a smoosh channel with
concurrency one can start arbitrarily many additional database compaction jobs.

To solve this problem, we added a check to see if a compaction PID exists for
a db in start_compact. But wee need to add another check because this check
is only for shard that comes off the queue. So the following can still occur:

1. Enqueue a bunch of stuff into channel with concurrency 1
2. Begin highest priority job, Shard1, in channel
3. Compaction finishes, discovers compaction file is behind main file
4. Smoosh-monitored PID for Shard1 exits, a new one starts to finish the job
5. Smoosh receives the 'DOWN' message, begins the next highest priority job,
   Shard2
6. Channel concurrency is now 2, not 1

This change adds another check into the 'DOWN' message so that it checks for
that specific shard. If the compaction PID exists then it means a new process
was spawned and we just monitor that one and add it back to the queue. The
length of the queue does not change and therefore we won’t spawn new
compaction jobs.

Testing recommendations

Related Issues or Pull Requests

Current PR is the porting of PR #3144 and #3150 to 3.x

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation