IsAlive is ambiguous because the connection may be dead and we do not
know it. It implies the possibility of a ping. IsClosed is clearer -- it
does not promise the connection is alive only that it hasn't been
closed.
This simplifies handling default values. Now there is no ambiguity
between a zero value and a default value. All default values are set by
ParseConfig and the user can modify them after the initial creation.
fixes#567
This is in preparation for a Begin / Tx interface that will similate
nested transactions with savepoints.
In addition, this passes the TxOptions struct by value and thereby
removes an allocation.