Refactor regression tests for more extensibility

Regression tests are still managed with scripts, except that now commands
are kicked by a psql client. This has the advantage to move output diff
evaluation outside the existing script and let pg_regress manage it.

Patch by Haishan Du, with some editorialization and tweaks by me.
Remove mandatory use of checksums and add wal_log_hints

This is to be in-line with Postgres 9.4 code.
Remove expected/pg_rewind.out

This output file is unnecessary.
Correct regression command in README

Regression tests should be kicked with installcheck when this module
is managed out of Postgres code tree.