async_wrap: use double, not int64_t, for async id

The number of ids is limited to 2^53-1 regardless of whether an int64_t
is used or not because JS is limited to a double. So to make conversion
simpler, track ids internally as a double. This will also make life
simpler when this is eventually exposed to JS via a Float64Array.

Rename AsyncWrap::get_uid() to AsyncWrap::get_id().

PR-URL: https://github.com/nodejs/node/pull/12892
Ref: https://github.com/nodejs/node/pull/11883
Ref: https://github.com/nodejs/node/pull/8531


Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>