Add support for querying sharedb meta fields

[jylauril]

Changes

This change adds support for making queries against the ShareDB doc's meta property.

There are two distinct changes in this PR:

1. Add a workaround for ShareDB's MemoryDB behavior of removing metadata from the snapshot before filtering is executed. Previously the snapshots that were passed in to _querySync did not have metadata unless explicitly defined in the query options, thus preventing any filtering to be done with the meta fields.

2. Use a property segment to check if the property should be considered a MONGO_DOC_PROPERTY. Previously the check was using the entire property name which prevented nesting the property paths such as _m.mtime.

Notes

There has been some discussion about refactoring this adapter to use snapshot casting instead of query casting. The changes in this PR are not addressing any of that, but is instead providing an interim implementation so the meta fields can be used for querying purposes.

[ericyhwang]

Looks good, thanks for the improvement!

Shame it wasn't feasible to do this without changing MemoryDB itself or duplicating the logic in a method override... MemoryDB wasn't intended to be super bulletproof.

[ericyhwang]

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