rdbms: add IDatabase::wasConnectionLoss() method

rdbms: add IDatabase::wasConnectionLoss() method

This takes the logic from wasErrorReissuable(), but puts it under
better name. The way that method was used, as well its comments,
were only about connection loss.

Make wasErrorReissuable() check if there was any error that
does not preclude the ability to retry. This matches the actual
name of the method.

Also improve some other related comments.

Change-Id: I68455d803afb2370897fecab0e79aadbb5d1a740