* fix(filer): don't disable the SQL idle connection pool when unconfigured
The mysql/mysql2/postgres stores called SetMaxIdleConns(maxIdle)
unconditionally, so an unset connection_max_idle (0) actively kept zero
idle connections - every query opened and closed a fresh connection
instead of reusing the pool.
Only apply the value when it's set; otherwise leave database/sql's
default idle pool of 2 in place.
* comments: shorten idle-pool note
* fix(filer): default the SQL idle pool via config, keep explicit 0 honored
Apply the idle-pool default at the config layer with SetDefault instead of
guarding the SetMaxIdleConns call. An absent connection_max_idle now reads
back as 2 (pool stays on), while an explicit 0 flows through to
SetMaxIdleConns(0) so operators can still disable idle pooling on purpose.