Revert commit e0492d4 "Ensure a query is canceled, if a thread is about
to be killed."

This is due to this discussion:
https://groups.google.com/d/topic/ruby-pg/5_ylGmog1S4/discussion

We were using rb_thread_call_without_gvl() with an ubf which canceled
the query currently being processed. This turned out to be incompatible
with possibly registered signal handlers (by Signal.trap).
A ubf is unconditionally invoked if ANY signal fires on a process (eg
even: USR1 or PROF) and Ruby has handlers registered. Cancelling
queries in those conditions does no make sense.

This commit re-enables Connection#async_exec based on the async API
for all ruby versions, because async_exec allowes cancelation of
queries by signals like Control+C.

Thanks to Sam Saffron for highlighting this issue.

--HG--
branch : revert_query_cancel_on_ubf
Add test case for compatibility with signal handlers. See 97c3ec2.

--HG--
branch : revert_query_cancel_on_ubf
Merged in larskanis/ruby-pg/revert_query_cancel_on_ubf (pull request #18)

Revert commit e0492d4 "Ensure a query is canceled, if a thread is about to be killed."
Update History file
Bump the patch version, set the date in History.
Added signature for changeset 22d57e3a2b37