Add query-specific uuid to enable finite metrics.

[funnylookinhat]

When trying to profile queries throughout an application, binding to query, query-response, and query-error isn't quite as useful as it could be because there's no way to uniquely identify individual queries that are sent off to the connection.

This adds a (hopefully appropriately) named field of __knexQueryUid to both raw and query-builder queries.

For example:

knex.on('query', function (obj) {
  // Start profiling obj.__knexQueryUid
});
knex.on('query-response', function (res, obj) {
  // Record metrics for obj.__knexQueryUid
});
knex.on('query-error', function (err, obj) {
  // Record metrics for obj.__knexQueryUid
});

This should hopefully resolve this feature request: #1502

I didn't see any tests specific to the events that are emitted when running queries - so I'm not sure if/where I should add tests for this PR.

[wubzz]

The tests for the events are awkwardly placed here, in case you wanted to write some.

[funnylookinhat]

@wubzz Ah thanks :) I've added what I think should be necessary now.

[funnylookinhat]

@wubzz Let me know if you have any feedback. Sorry to spam the PR - but I couldn't get the tests working locally so I just abused Travis. :-P