Stop leaking an extra dbh during server startup

RT::Handle->CheckIntegrity is called during rt-server startup, and its
return value is is stored in $integrity.  Additionally, $RT::Handle is
explicitly destroyed, to ensure that the database is disconnected before
forking.

Unfortunately, this replies on the dbh going out of scope, and
disconnecting during its DESTROY.  As the dbh is stored in the
$integrity variable in addition to $RT::Handle->dbh, it never goes out
of scope, and is thus never disconnected.  For fastcgi deployments, this
causes every fastcgi process to hold open an extra database handle;
standalone servers only ever gain one extra handle.

As no callsite of CheckIntegrity relies on the $dbh return value, simply
return true.  This allows the database handle to be disconnected using
$RT::Handle->dbh(undef).
Explicitly disconnect the dbh before forking, if it exists

This remoes the reliance on disconnect-on-DESTROY, and ensures that the
database handle is disconnected even if outstanding references to
$RT::Handle->dbh still exist.
Merge branch '4.2/fewer-active-handles' into 4.2-trunk