Unify timestamps of query cache keys

[tillkruss]

This is a followup to #8615. Currently query cache keys are un-parseable and they should be unified.

<hash>:0.91192600-1743003204:0.90454100-1743003204
<hash>:0.91192600-17430032040.90454100-1743003204
<hash>:0.90454100-1743003204

I propose this new format, where the primary cache group is always last:

<hash>:<2nd-group>+<group>
<hash>:<group>

<hash>:0.91192600-1743003204+0.90454100-1743003204
<hash>:0.90454100-1743003204

Trac ticket: https://core.trac.wordpress.org/ticket/63195

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[tillkruss]

Closing in favor of #8728.