CRM_Core_Key - Improve entropy of "privateKey"

CRM_Core_Key - Improve entropy of "privateKey"

In PHP 4/5, there was no good, universal source of entropy.  The old code
mitigated this by aggregating mediocre sources.  On my system, it appears
to be roughly:

* 2^31 for each `mt_rand()`
* 10^8 =~ 2^26 for each `uniqid(...TRUE)` (after discounting the non-random right half of the uniqid).

So that's ~114 bits (albeit low-quality bits).

In PHP 7, the docs describe `random_bytes()` as "generat[ing] cryptographically secure pseudo-random bytes."