make sql db wait for connections instead of returning 'too many connections'

[rdallman]

we need to probably figure out how to make this stop across all the various backends, we would prefer for them to wait and then possibly get hit with a timeout instead of just failing.

see #125 for error (now masked to user, still happening)

wrt mysql, it is mysql returning this error (not the database/sql package). see golang/go#4805 for context on the pool, this package will wait for a free connection & we are using this feature. we need a better way to sync our local max_conns with the mysql in use's max_conns as these 2 can be different numbers and is the source of this error.

[rdallman]

\cc @denismakogon

[rdallman]

mysql error="invalid connection" too. this appears to be related to go-sql-driver/mysql#302 (we're at the commit where this was merged when this popped up).

[rdallman]

this was mostly related to the mysql bindings, we're locked on an old version until further notice, so closing since this is irrelevant