Introduce local hash table for lock state, as per recent proposal.

PROCLOCK structs in shared memory now have only a bitmask for held
locks, rather than counts (making them 40 bytes smaller, which is a
good thing).  Multiple locks within a transaction are counted in the
local hash table instead, and we have provision for tracking which
ResourceOwner each count belongs to.  Solves recently reported problem
with memory leakage within long transactions.