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
Added tag v0.17.1 for changeset 78846e47d87b
Fix typo in documentation.
Convert specs to expect syntax for RSpec 3

--HG--
rename : spec/lib/helpers.rb => spec/helpers.rb
Merge with 6c2444dc63e1