Allow `max_connections` to be set in RedisSettings

[danbox]

Default settings for Redis is an "unbounded" (i.e. 2**31) connection pool.

This means that misuse of the client can lead to the consumption of every available connection in Redis locking out additional clients and there is no way in arq to modify that setting without monkey patching or rolling your own create_pool function. This PR exposes a setting for connection pool size.

Allowing setting max_connections allows for a safeguard to preemptively thwart workers that are running amuck with client creation.

[codecov[bot]]

Codecov Report

Merging #406 (c14d19a) into main (94cd878) will increase coverage by 2.10%. Report is 4 commits behind head on main. The diff coverage is 100.00%.

:exclamation: Current head c14d19a differs from pull request most recent head 8678b5e. Consider uploading reports for the commit 8678b5e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
+ Coverage   96.27%   98.38%   +2.10%     
==========================================
  Files          11       22      +11     
  Lines        1074     2106    +1032     
  Branches      209      364     +155     
==========================================
+ Hits         1034     2072    +1038     
+ Misses         19       14       -5     
+ Partials       21       20       -1     

Files	Coverage Δ
arq/connections.py	95.23% <100.00%> (+5.17%)	:arrow_up:

... and 14 files with indirect coverage changes

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[samuelcolvin]

thanks so much.

[samuelcolvin]

v0.26.0b1 is released, please try it, I'll release v0.26 at the end of the week, see #441.