Re: [HACKERS] make async slave to wait for lsn to be replayed

Alexander, thank you for your review and pointing this issues. According to
them I made some fixes and rebase all patch.

But I can`t repeat your ERROR. Not with hot_standby_feedback = on nor 
hot_standby_feedback = off.

master: create table test as (select i from generate_series(1,10000) i);
slave conn1: select pg_wal_replay_pause();
master: delete from test;
master: vacuum test;
master: select pg_current_wal_lsn();
slave conn2: select pg_wait_lsn('the value from previous query'::pg_lsn, 0);
slave conn1: select pg_wal_replay_resume();
slave conn2: ERROR: canceling statement due to conflict with recovery
DETAIL: User query might have needed to see row versions that must be removed.

Also I use little hack to work out of snapshot similar to SnapshotResetXmin.

Patch rebased and ready for review.