shared-arena: shared arena, closes #36
Allocate slab arena in shared memory to reduce pages splits during
snapshotting.
Adaptation of patch made by Yuri Nevinitsin @nevinitsin.
shared-arena: review fixes, slab_q moved to separate file.
shared-arena: make arena snapshot-version aware.

On snapshot, increase snapshot version id (u32),
save current snapshot version within a tuple.

On tuple free, check if tuple version is equal to current
snapshot id, if so do direct free, otherwise append tuple
to the delayed free list.
gh-47: (shared-arena) review fixes

Add comments, update the manual.
Merge branch 'shared-arena' into stable
gh-47: update wal/oom.test

Tuples are got a big bigger with shared-arena patch (4 bytes
per tuple overhead).

So we exhaust the current arena in out-of-memory test a bit
faster than before.