A pre-requisite patch for Lua stored procedures: fiber->pool.

A pre-requisite patch for Lua stored procedures:

in some cases, it would be nice to create a Lua context
in connection and destroy it when a connection ends.

Unfortunately, right now fiber connections are stateless:
there is on fiber constructor, destructor, or non-global fiber
state.

Change that by introducing fiber->pool, an allocation
pool in which memory lives till a fiber is destroyed
or becomes a zombie.

We already have fiber->pool, which life cycle is different:
it is created when a fiber starts, destroyed when a fiber
ends, but also is garbage-collected once in a while.

Rename this pool to fiber->gc_pool, to allow using
name 'fiber->pool for something else.

This patch only performs a rename and does nothing else.