Move session table initialization into session file.

This moves all the code working on the session table to that file.
Fix offset calculation for session entries.

The session entry size isn't known on compile time, so we must
actually calculate the offset at runtime.

This fixes a nasty bug where we would randomly overwrite session
entries.
Fix am_cache_env_fetch_first.

This function is supposed to return the value of the entry we are
looking for, but instead it returned the name. Fix it to return the
value.

Also, fix exit condition on for-loop.

This fixes NameID-based logout.
Add NEWS-file for 0.9.0.
Version 0.9.0.