Show size of DSAs and dshashes in pg_dsm_registry_allocations.

Show size of DSAs and dshashes in pg_dsm_registry_allocations.

Presently, this view reports NULL for the size of DSAs and dshash
tables because 1) the current backend might not be attached to them
and 2) the registry doesn't save the pointers to the dsa_area or
dshash_table in local memory.  Also, the view doesn't show
partially-initialized entries to avoid ambiguity, since those
entries would report a NULL size as well.

This commit introduces a function that looks up the size of a DSA
given its handle (transiently attaching to the control segment if
needed) and teaches pg_dsm_registry_allocations to use it to show
the size of successfully-initialized DSA and dshash entries.
Furthermore, the view now reports partially-initialized entries
with a NULL size.

Reviewed-by: Rahila Syed <[email protected]>
Reviewed-by: Robert Haas <[email protected]>
Reviewed-by: Chao Li <[email protected]>
Discussion: https://postgr.es/m/aSeEDeznAsHR1_YF%40nathan