feat: introduce pool acquisition timeout (fixes #2348)

The configuration option db-pool-acquisition-timeout
specifies the time in seconds to wait for the pool to
free up a connection slot. Otherwise, a 504 error is
returned. By default, there is no timeout.

test/io/configs/expected/no-defaults-with-db.config

@@ -5,6 +5,7 @@ db-extra-search-path = "public,extensions,private"
 db-max-rows = 1000
 db-plan-enabled = true
 db-pool = 1
+db-pool-acquisition-timeout = 10
 db-pre-request = "test.custom_headers"
 db-prepared-statements = false
 db-root-spec = "root"