PHP 7.3: use hrtime() instead of microtime()

[Llbe]

Guzzle version(s) affected: 6.3.3

Description
Consider switching from microtime() to hrtime(), available in PHP 7.3. It provides a monotonic timestamp rather than a realtime timestamp.

How to reproduce
Using microtime() is unsafe e.g. on a DST crossover or whenever the system time is adjusted, since the timestamps aren't relative to each other.

It is used in the CurlMultiHandler and the on_stats option.

Apart from needing to keep support for < 7.3, the change here should be without a BC break.

[Llbe]

Basically function_exists('hrtime') ? hrtime(true) / 1e9 : microtime(true) could be used (an internal wrapper would be preferable of course). See https://3v4l.org/BATQ7

[Dzhuneyt]

Fixed in #2242