feat: Add cancelQuery() to PGConnection public interface

[sehrope]

Adds cancelQuery() to the extended public interface PGConnection to allow for issuing of cancels directly to connections rather than having to keep track of individual statements. The internals of the driver and the backend server already operate at a connection level (not a statement level) and this patch only exposes
the existing cancelQuery() method publicly.

Exposing the cancelQuery() publicly is mainly a convenience thing as it's already part of all the concrete implementations. This is how how the server handles cancellation (i.e. at the connection, not statement, level) so I don't foresee any long term baggage associated with exposing this. The backend functionality is not going away.

There's been at least one other request for this in the past: #1084

Any thoughts on merging this in?

[codecov-io]

Codecov Report

Merging #1157 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1157      +/-   ##
============================================
- Coverage     68.94%   68.93%   -0.02%     
+ Complexity     3775     3774       -1     
============================================
  Files           170      170              
  Lines         15757    15757              
  Branches       2577     2577              
============================================
- Hits          10864    10862       -2     
- Misses         3658     3659       +1     
- Partials       1235     1236       +1

[AppVeyorBot]

✅ Build pgjdbc 1.0.201 completed (commit db109455e3 by @sehrope)