RE: Performance TODO items

> New TODO entries are:
>
> * Order duplicate index entries by tid

In other words - add tid to index key: very old idea.

> * Add queue of backends waiting for spinlock

We shouldn't mix two different approaches for different
kinds of short-time internal locks - in one cases we need in
light lmgr (when we're going to keep lock long enough, eg for IO)
and in another cases we'd better to proceed with POSIX' mutex-es
or semaphores instead of spinlocks. Queueing backends waiting
for spinlock sounds like nonsense - how are you going to protect
such queue? With spinlocks? -:)

Vadim