CBL-6282: Replicator starts up slow for big database

[callumbirks]

Cherry-pick 711f886:

Optimize unsorted enumeration of BothKeyStore (#1858)

Use a different algorithm to enumerate a BothKeyStore when the sortOption is kUnsorted: just enumerate the live docs first, then the deleted ones. This removes the need to order the underlying enumerations, which in turn allows SQLite to use indexes when onlyConflicts is set. This should fix CBL-4506

[cbl-bot]

Code Coverage Results:

Type	Percentage
branches	68.45
functions	80.11
instantiations	33.97
lines	79.02
regions	76.23