- 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

Documentation fix-up: "its" vs. "it's"

Removed ungrammatical apostrophes from documentation, replacing
"it's" with "its" where appropriate (but in a few cases with "it is"
when that read better).

While doing that, I also fixed a couple of minor typos etc.
as I noticed them.

a connection invalidation could occur within an already critical section
like a connection.close(); ultimately, these conditions are caused
by the change in :ticket:`2907`, in that the "reset on return" feature
calls out to the Connection/Transaction in order to handle it, where
"disconnect detection" might be caught.  However, it's possible that
the more recent change in :ticket:`2985` made it more likely for this
to be seen as the "connection invalidate" operation is much quicker,
as the issue is more reproducible on 0.9.4 than 0.9.3.

Checks are now added within any section that
an invalidate might occur to halt further disallowed operations
on the invalidated connection.  This includes two fixes both at the
engine level and at the pool level.   While the issue was observed
with highly concurrent gevent cases, it could in theory occur in
any kind of scenario where a disconnect occurs within the connection
close operation.
fixes #3043
ref #2985
ref #2907

- add some defensive checks during an invalidate situation:
1. _ConnectionRecord.invalidate might be called twice within finalize_fairy
if the _reset() raises an invalidate condition, invalidates, raises and then
goes to invalidate the CR.  so check for this.
2. similarly within Conneciton, anytime we do handle_dbapi_error(), we might become invalidated.
so a following finally must check self.__invalid before dealing with the connection
any futher.

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

it for DELETE would fail to target the correct row for DELETE.
Then to compound matters, basic "number of rows matched" checks were
not being performed.  Both issues are fixed, however note that the
"rows matched" check requires so-called "sane multi-row count"
functionality; the DBAPI's executemany() method must count up the
rows matched by individual statements and SQLAlchemy's dialect must
mark this feature as supported, currently applies to some mysql dialects,
psycopg2, sqlite only. fixes #3006
- Enabled "sane multi-row count" checking for the psycopg2 DBAPI, as
this seems to be supported as of psycopg2 2.0.9.

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

implementation allows an event handler to redefine the specific mechanics
by which an arbitrary dialect invokes execute() or executemany() on a
DBAPI cursor.  The new events, at this point semi-public and experimental,
are in support of some upcoming transaction-related extensions.

after one or more :class:`.Connection` objects have been created
(such as by an orm :class:`.Session` or via explicit connect)
and the listener will pick up events from those connections.
Previously, performance concerns pushed the event transfer from
:class:`.Engine` to  :class:`.Connection` at init-time only, but
we've inlined a bunch of conditional checks to make this possible
without any additional function calls. fixes #2978