a workaround commit (wip at gpusort.c)
bugfix: concurrent query execution and unexpected tc_head release

* tcache_get_tchead and tcache_try_create_tchead didn't take
  tc_common->lock prior to tc_head hash slot access.
* we implemented tcache creator process should not put tcache-
  head to retain refcnt==1; it leads code complexity and
  potential bug.

We fixed up tcache creation code to set refcnt=2 as initial value.
Then tcache shall be put by all the process, once it grabbed.
Also, tc_common->lock is acquired on cache search.