make SAFE_METHODS a set

Slightly faster to check if SAFE_METHODS.include?(x)
When slow queries time out, re-raise the error

As opposed to rescueing the error and retrying on master.
better logging on mysql errors

Per @kastner's comment:
https://github.com/kickstarter/slave_pools/pull/1#discussion_r3228076
Merge pull request #1 from kickstarter/query_timeouts

Better handle slow queries