Support batching for prepared statements that return rows.

With this change, PgJDBC calculates the estimated return size for the result
row, assuming that each batch item returns one row for getGeneratedKeys() use.

Instead of forcing batching off and sending individual queries, it then queues
enough binds and executes to fit safely within the receive buffer and server's
send buffer before sending a sync and reading the result buffer.

Batching is still forced off when the result set size cannot be estimated (e.g.
due to unbounded 'text' columns, use of arrays in the result set, etc).

This is a minimal workaround for github issue #195, though it doesn't prevent
deadlocks that can already occur due to #194.