Fix the deadlock bug in gen_miRNA_loci_nopredict

The deadlock problem was not solved successfully in commit 4008981. The
deadlock happens because the underlining pipe used to implement the
queue is full and is blocked, thus there is no chance to execute the
join method in the main process. See here for detail and workaround:
https://mail.python.org/pipermail/python-list/2009-October/553853.html