libthr: Cache recent pshared_key

[aokblast]

Summary

For a pshare key, we can actually cache the last one as a fast path
instead of trapping into kernel directly. It makes sense as a lock and
unlock should be a pair on the same key and at the same time. For
example, lock(A) -> unlock(A) -> lock(B) -> unlock(B). Each unlock is
a hit and refresh later (50% hit).
For a single pshare lock, it is evne a 100% hit as the key never
changed.
For nested lock lock(A) -> lock(B) -> lock(C) -> unlock(C) ->
unlock(B) -> unlock(A). It only cache the last one. However, we should
really think about rewritting this logic as it is a pshare lock shared
by many processes and nested should be uncommon.