[WIP] Add support for recursive locks and try_lock

[janciesko]

This implements a recursive lock using FEBs. The idea is that qthreade_read_FE becomes re-entrant if qthread_read_FE is called from qthread_lock and qthread_lock_init has been called on the same address setting is_recursive to true. Analogously, qthread_unlock calls qthead_fill which decrements a counter of a recursive lock. If the counter reaches zero, blocked waiters (EFQ) are unblocked.

Uses of other FEB calls over recursive locks are unsupported and we could error out. Currently they have no effect but could lead to programming errors if programmer expects FEB synchronization over an address used for a recursive lock.

Todo

• Add unit test
• Error out in incorrect use
• Update qthread_feb_blocker_func to use proper func name

With this infrastructure, we should be able to support ULTs from Open MPI.
For Open MPI, we'd need then these changes:
janciesko/ompi@daef600
janciesko/ompi@bab853b

[janciesko]

This implementation is not going to fly. The problem is that unlike locks in ptheads, FEBs do not poll a state but generate the dependency graph at instantiation and thus cannot ignore conditions based on ownership (runtime property). At runtime, we'd need to modify the graph in order to ignore prod-cons but the notion of recursiveness is lost and we cannot associate inner producers to inner consumers.

[janciesko]

Closing in favor of #112