Cache asyngpg prepared statements

Cache asyngpg prepared statements

Enhanced the performance of the asyncpg dialect by caching the asyncpg
PreparedStatement objects on a per-connection basis. For a test case that
makes use of the same statement on a set of pooled connections this appears
to grant a 10-20% speed improvement.  The cache size is adjustable and may
also be disabled.

Unfortunately the caching gets more complicated when there are
schema changes present.  An invalidation scheme is now also added
to accommodate for prepared statements as well as asyncpg cached OIDs.
However, the exception raises cannot be prevented if DDL has changed
database structures that were cached for a particular asyncpg
connection.  Logic is added to clear the caches when these errors occur.

Change-Id: Icf02aa4871eb192f245690f28be4e9f9c35656c6