[3.0.1 backport] Implementation for supporting top-level document properties with an underscore prefix

Allow documents to have top-level document properties that start with an underscore ("_") outside of the reserved keys. This means that documents can be created, replicated, and imported to use this previously disallowed top-level document property prefix.

The reserved keys are:

_sync
_id
_rev
_deleted
_attachments
_revisions
_exp
_purged
_removed

sync will be a reserved prefix, allowing for future internal properties to be created without breaking existing user deployments.

This will need to be unit tested to make sure import, API, and ISGR works as intended with these prefixes and make sure that using reserved prefixes is disallowed.

No extra QE test cases or considerations are needed so only normal regression tests will have to be ran.