Modify the LargeObject module to use non-blocking calls to libpq

[lpsmith]

As a follow up to #25, the LargeObject module still uses blocking C calls. Libpq does not offer non-blocking versions of these functions, however, it's entirely possible to avoid these calls and use sendQuery, getResult, and friends to accomplish the same task using non-blocking C calls.

Also, this would allow us to implement out-of-box support for 64-bit large objects (c.f. lpsmith/postgresql-libpq#10) without worrying about backwards-compatible linking with older versions of libpq.

It may well be worth deprecating the large object bindings in postgresql-libpq after this issue has been implemented. I don't think there are any hackage packages other than postgresql-simple that use postgresql-libpq's large object bindings, and those who want to use large objects without postgresql-simple would be free to steal and adapt the new bindings.