Add a WAIT option to DROP_REPLICATION_SLOT

Add a WAIT option to DROP_REPLICATION_SLOT

Commit 9915de6c1cb2 changed the default behavior of
DROP_REPLICATION_SLOT so that it would wait until any session holding
the slot active would release it, instead of raising an error.  But
users are already depending on the original behavior, so revert to it by
default and add a WAIT option to invoke the new behavior.

Per complaint from Simone Gotti, in
Discussion: https://postgr.es/m/CAEvsy6Wgdf90O6pUvg2wSVXL2omH5OPC-38OD4Zzgk-FXavj3Q@mail.gmail.com