CBL-3799: More REST listener collection work

[snej]

• GET /db includes a scopes property containing all the scopes. Every scope contains its collections, every collection contains its doc_count and update_seq, and all of them were coming from St. Ives.
• Implemented PUT /db.scope.coll and DELETE /db.scope.coll .
• C4ListenerConfig controls whether collections can be created/deleted.
• More error handling when parsing the scope/collection from a request.
• Unit tests

Also a separate commit that fixes a C4Database crash that was exposed by the other commit.

[snej]

The Jenkins tests failed because I forgot to push the LiteCore-EE repo changes. Retrying...

[cbl-bot]

Code Coverage Results:

Type	Percentage
functions	79.71
instantiations	33.52
lines	79.73
regions	75.44