Rename MAX_NAMED_TRANCHES to MAX_USER_DEFINED_TRANCHES

Rename MAX_NAMED_TRANCHES to MAX_USER_DEFINED_TRANCHES

The "named tranches" term is a little confusing. In most places it
refers to tranches requested with RequestNamedLWLockTranche(), even
though all built-in tranches and tranches allocated with
LWLockNewTrancheId() also have a name. But in MAX_NAMED_TRANCHES, it
refers to tranches requested with either RequestNamedLWLockTranche()
or LWLockNewTrancheId(), as it's the maximum of all of those in total.

The "user defined" term is already used in
LWTRANCHE_FIRST_USER_DEFINED, so let's standardize on that to mean
tranches allocated with either RequestNamedLWLockTranche() or
LWLockNewTrancheId().

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://www.postgresql.org/message-id/47aaf57e-1b7b-4e12-bda2-0316081ff50e@iki.fi