querystring: fix empty pairs and optimize parse()

This commit fixes handling of empty pairs that occur before the end
of the query string so that they are also ignored.

Additionally, some optimizations have been made, including:

* Avoid unnecessary code execution where possible
* Use a lookup table when checking for hex characters
* Avoid forced decoding when '+' characters are encountered and we
are using the default decoder

Fixes: https://github.com/nodejs/node/issues/10454
PR-URL: https://github.com/nodejs/node/pull/11234


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Nicu Micleușanu <micnic90@gmail.com>