Re: Patch: fix lock contention for HASHHDR.mutex

Here is a clean version of the patch. Step 1 is an optimization. Step 2
refactors this:

HTAB *
ShmemInitHash(const char *name, /* table string name for shmem index */
- long init_size, /* initial table size */
+ long init_size, /* initial table size XXX ignored,
refactor! */

As I understand there is no performance degradation:

Before
======

Core i7, pgbench -j 8 -c 8 -T 50 pgbench:

```
number of transactions actually processed: 14315
latency average: 27.945 ms
latency stddev: 44.920 ms
tps = 286.097043 (including connections establishing)
tps = 286.127642 (excluding connections establishing)
```

60-core server, pgbench -j 64 -c 64 -T 50 pgbench:

```
number of transactions actually processed: 176127
latency average: 18.162 ms
latency stddev: 23.861 ms
tps = 3521.069672 (including connections establishing)
tps = 3522.166706 (excluding connections establishing)
```

After
=====

Core i7, pgbench -j 8 -c 8 -T 50 pgbench:

```
number of transactions actually processed: 14212
latency average: 27.984 ms
latency stddev: 43.250 ms
tps = 284.038588 (including connections establishing)
tps = 285.112772 (excluding connections establishing)
```

60-core server, pgbench -j 64 -c 64 -T 50 pgbench:

```
number of transactions actually processed: 172135
latency average: 17.767 ms
latency stddev: 22.692 ms
tps = 3590.410302 (including connections establishing)
tps = 3594.607165 (excluding connections establishing)
```