Prepare DSM registry for upcoming changes to LWLock tranche names.

Prepare DSM registry for upcoming changes to LWLock tranche names.

A proposed patch would place a limit of NAMEDATALEN-1 (i.e., 63)
bytes on the names of dynamically-allocated LWLock tranches, but
GetNamedDSA() and GetNamedDSHash() may register tranches with
longer names.  This commit lowers the maximum DSM registry entry
name length to NAMEDATALEN-1 bytes and modifies GetNamedDSHash() to
create only one tranche, thereby allowing us to keep the DSM
registry's tranche names below NAMEDATALEN bytes.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/aKzIg1JryN1qhNuy%40nathan