Set database poolsize via rails_max_threads

[bensheldon]

Introduced in #23057 was an environment variable RAILS_MAX_THREADS that is used to set the number of threads in puma, the default web server. The default value for this environment variable is 5 because that matches the default number of ActiveRecord threads. A mismatch between the database poolsize and the puma thread-count results in ActiveRecord::ConnectionTimeoutError.

This PR proposes to also reuse the RAILS_MAX_THREADS when setting the database pool size to better ensure that the values are in sync.

[maclover7]

cc @schneems

[schneems]

I'm for this, it keeps the default the same as it always was, but plays nice with the puma config. Can you rebase this to 1 commit?

[bensheldon]

@schneems I've rebased to a single commit.

[schneems]

Thanks!