Improve and fix connection timeout and total retry timeout. (#528)

Previously, when a connection timeout occurred with the default connection
timeout set to pgconnect_timeout (10 seconds), there were no retries. This
was because the total retry timeout (maxT) passed to `pgsql_set_retry_policy`
in `pgsql_set_interactive_retry_policy` was also set to pgconnect_timeout,
which had already elapsed during the initial connection attempt.

To address this issue, the maxT value has been updated to
POSTGRES_PING_RETRY_TIMEOUT. This change also means that maxT cannot be
configured by altering PGCONNECT_TIMEOUT. If one wants to configure maxT
they can change the values in defaults.h or we can make it configurable
through some environment variable in future.

After all this change `pgconnect_timeout` seems redundant so removed it.