xlock: Use TID as lock owner

Use TID as lock owner of the xseg locks.
Remove the provided owner argument and use a (cached) TID value as the lock
owner.
xq: Remove 'who' argument

Remove obsolete 'who' arguement, which was used as an owner for the
corresponding xlock structure. Since xlocks now creates its own owner, theres is
no need for providing an 'owner' arguement on xq API.
xpool: Remove 'who' argument