Improve monitoring of shared memory allocations

Hi,

The 0001* patch improved the accounting for the shared memory allocated for
a
hash table during hash_create.
pg_shmem_allocations tracks the memory allocated by ShmemInitStruct, which,
for shared hash tables, only covers memory allocated for the hash
directory
and control structure via ShmemInitHash. The hash segments and buckets
are allocated using ShmemAllocNoError, which does not attribute the
allocation
to the hash table and also does not add it to ShmemIndex.

Therefore, these allocations are not tracked in pg_shmem_allocations.
To improve this, include the allocation of segments and buckets in the
initial
allocation of the shared memory for the hash table, in ShmemInitHash.

This will result in pg_shmem_allocations representing the total size of the
initial
hash table, including all the buckets and elements, instead of just the
directory
size.

Like ShmemAllocNoError, the shared memory allocated by ShmemAlloc is not
tracked by pg_shmem_allocations.
The 0002* patch replaces most of the calls to ShmemAlloc with
ShmemInitStruct
to associate a name with the allocations and ensure that they get tracked
by
pg_shmem_allocations.

I observed an improvement in total memory allocation by consolidating
initial shared
memory allocations for the hash table. For ex. the allocated size for the
LOCK hash
hash_create decreased from 801664 bytes to 799616 bytes. Please find the
attached
patches, which I will add to the March Commitfest.

Thank you,
Rahila Syed