SQLite used one connection for everything, so request lookups queued
behind the per-minute stats queries, dashboard pages and eviction scans.
Writes keep the single connection. Get, Select, Query and QueryRow now go
to a pool of query_only connections (GOMAXPROCS, 2 to 8), which WAL mode
lets run alongside the writer. A write sent to that pool fails.
Pragmas move into the DSN so every connection gets them. Postgres and
in-memory SQLite keep one pool for both.
