* client/allocdir: use an interface in place of AllocDir structs
This PR replace *allocdir.AllocDir with allocdir.Interface such that we
may eventually have another implementation of alloc directories. This is
in support of the exec2 driver, which will need an implementation of the
alloc directory incompatibile with the current version.
* use rlock
Some of the context uses in TR hooks are useless (Killed during Stop
never seems meaningful).
None of the hooks are interruptable for graceful shutdown which is
unfortunate and probably needs fixing.
