Consistently spell a replication slot's name as slot_name.

Previously there's been a mix between 'slotname' and 'slot_name'. It's
not nice to be unneccessarily inconsistent in a new feature. As a post
beta1 initdb now is required in the wake of eeca4cd35e, fix the
inconsistencies.
Most the changes won't affect usage of replication slots because the
majority of changes is around function parameter names. The prominent
exception to that is that the recovery.conf parameter
'primary_slotname' is now named 'primary_slot_name'.
Add defenses against running with a wrong selection of LOBLKSIZE.

It's critical that the backend's idea of LOBLKSIZE match the way data has
actually been divided up in pg_largeobject.  While we don't provide any
direct way to adjust that value, doing so is a one-line source code change
and various people have expressed interest recently in changing it.  So,
just as with TOAST_MAX_CHUNK_SIZE, it seems prudent to record the value in
pg_control and cross-check that the backend's compiled-in setting matches
the on-disk data.

Also tweak the code in inv_api.c so that fetches from pg_largeobject
explicitly verify that the length of the data field is not more than
LOBLKSIZE.  Formerly we just had Asserts() for that, which is no protection
at all in production builds.  In some of the call sites an overlength data
value would translate directly to a security-relevant stack clobber, so it
seems worth one extra runtime comparison to be sure.

In the back branches, we can't change the contents of pg_control; but we
can still make the extra checks in inv_api.c, which will offer some amount
of protection against running with the wrong value of LOBLKSIZE.
Fix off-by-one in decoding causing one-record events to be skipped.

A ReorderBufferTransaction's end_lsn, the sentPtr advocated by
walsender keepalive messages, and the end location remembered by the
decoding get_*changes* SQL functions all use the location of the last
read record + 1. I.e. the LSN points to the beginning of the next
record. That cannot realistically be changed without changing the
replication protocol because that's how keepalive messages have worked
since 9.0.
The bug is that the logic inside the snapshot builder, which decides
whether a transaction's contents should be decoded, assumed the start
location would point towards the last byte of the last record. The
reason this didn't actually cause visible problems is that currently
that decision is only made for commit records. Since interesting
transactions always have at least one additional record - containing
actual data - we'd never skip a transaction.
But if there ever were transactions, or other events, with just one
record containing important information, we'd skip them after stopping
and restarting logical decoding.