Minor lmgr code cleanup

Try to reduce confusion about what is a lock method identifier, a lock
method control structure, or a table of control structures.

. Use type LOCKMASK where an int is not a counter.

. Get rid of INVALID_TABLEID, use INVALID_LOCKMETHOD instead.

. Use INVALID_LOCKMETHOD instead of (LOCKMETHOD) NULL, because
LOCKMETHOD is not a pointer.

. Define and use macro LockMethodIsValid.

. Rename LOCKMETHOD to LOCKMETHODID.

. Remove global variable LongTermTableId in lmgr.c, because it is
never used.

. Make LockTableId static in lmgr.c, because it is used nowhere else.
Why not remove it and use DEFAULT_LOCKMETHOD?

. Rename the lock method control structure from LOCKMETHODTABLE to
LockMethodData. Introduce a pointer type named LockMethod.

. Remove elog(FATAL) after InitLockTable() call in
CreateSharedMemoryAndSemaphores(), because if something goes wrong,
there is elog(FATAL) in LockMethodTableInit(), and if this doesn't
help, an elog(ERROR) in InitLockTable() is promoted to FATAL.

. Make InitLockTable() void, because its only caller does not use its
return value any more.

. Rename variables in lock.c to avoid statements like
LockMethodTable[NumLockMethods] = lockMethodTable;
lockMethodTable = LockMethodTable[lockmethod];

. Change LOCKMETHODID type to uint16 to fit into struct LOCKTAG.

. Remove static variables BITS_OFF and BITS_ON from lock.c, because
I agree to this doubt:
* XXX is a fetch from a static array really faster than a shift?

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All 93 tests passed.
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Servus
Manfred