Add a per threadvars thread local thread id, that starts at 0 and increments for each thread.
pool: add error msgs and improve memory layout
pool: add api for per thread pools

This API is a wrapper around the regular pools where the thread pools
are arrays of locks+pools.
pool: add error msgs and improve memory layout
Stream: use per thread ssn pool

Use per thread pools to store and retrieve SSN's from. Uses PoolThread
API.

Remove max-sessions setting. Pools are set to unlimited, but TCP memcap
limits the amount of sessions.

The prealloc_session settings now applies to each thread, so lowered the
default from 32k to 2k.
Stream: fix unittests after ssn pool changes.
Improve memory cleanup in some unittests