add Synchronized::withLock() methods

add Synchronized::withLock() methods

Summary:
Add withLock() and related methods for invoking a lambda function while the
lock is held.  This is sometimes more convenient than opening a new scope and
using lock().

withLock() also retains some of the benefits of the SYNCHRONIZED macro in that
it forces users to put their critical section code in a new scope, making the
critical section more visibly distinct in the code.  This also encourages users
to only put necessary work inside the critical section, and do to other work
once the lock is released.

This also adds a LockedGuardPtr class, which is a slightly cheaper version of
LockedPtr.  The relationship between LockedGuardPtr and LockedPtr is very much
like that between std::lock_guard and std::unique_lock.  It saves a branch in
the destructor, and in the case of std::mutex it also saves a small amount of
storage space (since LockedPtr is specialized for std::mutex to also store a
std::unique_lock).

Reviewed By: yfeldblum, djwatson

Differential Revision: D3530368

fbshipit-source-id: 72a4f457b3f18e8e8f4cc6713218f6882bb89818