Default MT progress queue

[lappazos]

Currently, the default mt tasks progress queue is locked pq. For 2 threaded cases they perform almost the same, but for a bigger number of threads, we need the Lock-free progress queue. need to decide what will be the default, or maybe some other mechanism besides the config flag.

ucc/src/core/ucc_context.c

Line 21 in 61da0e9

{"LOCK_FREE_PROGRESS_Q", "0",