Re: pg_serial early wraparound

The following review has been posted through the commitfest application:
make installcheck-world: tested, passed
Implements feature: tested, passed
Spec compliant: not tested
Documentation: not tested

The patch doesn't break anything in regression tests and does the code cleanup.
As far as I understand, the removed code was dead, since SLRU size is large enough
and the wraparound, described in the message is impossible.
So I mark it as Ready For Committer.

I didn't manage to repeat the attached test, though.
Server doesn't start after xid reset. It throws an error:

server stopped
========== setting next xid to 1073741824 =========
Write-ahead log reset
waiting for server to start....2018-03-12 14:18:59.551 MSK [16126] LOG: listening on IPv4 address "127.0.0.1", port 5432
2018-03-12 14:18:59.625 MSK [16126] LOG: listening on Unix socket "/tmp/.s.PGSQL.5432"
2018-03-12 14:18:59.764 MSK [16127] LOG: database system was shut down at 2018-03-12 14:18:59 MSK
2018-03-12 14:18:59.802 MSK [16127] FATAL: could not access status of transaction 10737418
2018-03-12 14:18:59.802 MSK [16127] DETAIL: Could not open file "pg_xact/000A": Нет такого файла или каталога.
2018-03-12 14:18:59.803 MSK [16126] LOG: startup process (PID 16127) exited with exit code 1
2018-03-12 14:18:59.803 MSK [16126] LOG: aborting startup due to startup process failure
2018-03-12 14:18:59.804 MSK [16126] LOG: database system is shut down