v19.0.0

• mysql_common was updated to v0.22.2,
• tests for blackbeam/mysql_async#107 was added,
• pool exhaustion was fixed by @SpaceManiac (#226),
• the net2 crate was removed in favor of the socket2 crate (#233),
• the Protocol trait was exported by @mguillemot-elt (#235),
• various fixes in documentation (thanks to @dbanty, @nbrady-techempower, @Elykz, @eldad)
• #244 and related issues were fixed (hopefully)