pgsql: reconnect robustness improved.
pgsql: Rebalance threads, cut away some fat and unnecessary mutexes.

The benchmark performance is now within spitting distance of the
old pgsql driver for the unbatched single queries.
For large batches (>20), the new driver beats everyone pants down.
For single batches (==1), the new driver is about 2% slower than
the old 7.8 version.
pgsql: Off-by-one error.