With workers enabled, every Synapse process (the main one and each
worker) maintains its own database connection pool of up to
matrix_synapse_database_cp_max (default: 10) connections. The previous
fixed limit of 500 left little headroom on setups running many workers,
where exhausting it manifests as degraded performance and, reportedly,
E2EE misbehavior.

Size the limit as a 200-connection baseline (for everything else that
talks to Postgres) plus cp_max connections per Synapse process, never
going below the previous value of 500. The stock worker presets stay at
500; setups that raise worker counts (or cp_max) get a limit that grows
accordingly. The new matrix_synapse_workers_total_count variable
reflects what the matrix_synapse_workers_*_count variables ask for; it
does not reflect a manually populated
matrix_synapse_workers_enabled_list, and such setups should override
postgres_max_connections themselves.

Fixes #4442

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>