4591: Avoid capturing critical exceptions in Websockets server.

Based on the current logs, the troubles we're currently hitting in
Websockets happen in push_events, where all the database work
happens.  These exceptions wrap PostgreSQL driver errors; they inherit
from StandardError, so they're being caught by the rescue block.
This commit re-raises those exceptions, which will cause the server to
crash (and presumably be restarted by a supervisor like runit).

We do sometimes see NoMemoryError, but the block to catch is in
ineffective because it usually manifests earlier in on_connect, when
the connection is first made.  In this case, Ruby's default exception
handling provides the behavior we want, so just remove the block.

In keeping with the theme of improved exception handling, I tightened
up the bad request detection.
4591: Websockets server fetches fewer logs at a time.

Most of the out of memory errors we're seeing happen in the PostgreSQL
driver, which runs out of space to store results.  Because Log records
are relatively large (holding two other records as JSON text),
fetching fewer in a batch should noticeably improve memory use.  I
don't expect this to end the crashing, though—it seems like the
Websockets server grows large for a variety of reasons.  Hopefully
this change will help make some of the others clearer.
Merge branch '4591-websockets-raise-oom-wip'

Closes #4591, #4701.