Typo fixes

in all cases unconditionally, the number of use cases that go beyond what
dbapi_error() is expecting has gone too far for an 0.9 release.
Additionally, the number of things we'd like to track is really a lot
more than the five arguments here, and ExecutionContext is really not
suitable as totally public API for this.   So restore dbapi_error
to its old version, deprecate, and build out handle_error instead.
This is a lot more extensible and doesn't get in the way of anything
compatibility-wise.

have been enhanced such that the function handler is now capable
of raising or returning a new exception object, which will replace
the exception normally being thrown by SQLAlchemy.
fixes #3076

- Fixed bug which would occur if a DBAPI exception
occurs when the engine first connects and does its initial checks,
and the exception is not a disconnect exception, yet the cursor
raises an error when we try to close it.  In this case the real
exception would be quashed as we tried to log the cursor close
exception via the connection pool and failed, as we were trying
to access the pool's logger in a way that is inappropriate
in this very specific scenario. fixes #3063

Found using: https://github.com/intgr/topy

1. make sure pool._invalidate() sets the timestamp up before
invalidating the target connection.  we can otherwise show how the
conn.invalidate() + pool._invalidate() can lead to an extra connection
being made.
2. to help with that, soften up the check on connection.invalidate()
when connection is already closed. a warning is fine here
3. add a mutex to test_max_overflow() when we connect, because the way
we're using mock depends on an iterator, that needs to be synchronized