Fix #2556 by keeping callback errors from interfering with cleanup

[prust]

@brianc and @charmander: This pull request fixes #2556 (and possibly also #1105) by ensuring that errors thrown in user-supplied callbacks do not prevent the node-postgres library from doing necessary cleanup.

This is important for assert-driven testing (such as mocha and jest). Without this fix, one test failure in a query() callback will create a cascade of subsequent test failures.

This implementation should:

• be in keeping with the rest of the library (nextTick() is already used a few times in client.js to defer exception handling)
• make minimal changes to the existing behavior (the behavior only changes if an exception is thrown -- and even then, the only difference is that the re-throwing of the exception is deferred to the next tick)
• include a regression test

For the repro, see #2556 (comment), and for a description of potential solutions and workarounds, see #2556 (comment).

[brianc]

Ah yeah this makes sense. Thanks for the PR! I'll get this merged in & released asap. 😄

[prust]

Thanks so much @brianc, I appreciate it!