DB connection recovery (and more!)

[eduardoboucas]

With this PR:

• API becomes capable of recovering from database connection failures (API does not recover from a temporary database connection interruption #327)
• The DELETE method throws a 404 (instead of 204) when deleting a non-existing document by ID. This applies to requests where the document ID is passed in the URL, not when in the body (e.g. DELETE /v1/db/collection/DOC-ID vs DELETE /v1/db/collection) (DELETE should return 404 when no documents match the supplied query #345)
• The POST/PUT methods throw a 404 when updating a non-existing document by ID. This applies to requests where the document ID is passed in the URL, not when in the body (e.g. PUT /v1/db/collection/DOC-ID vs PUT /v1/db/collection)

Tests:

• API boots with database unavailable and recovers when it reconnects

• API boots with database available and recovers from subsequent failures

• RESTful DELETE on non-existing documents returns a 404

• RESTful POST/PUT on non-existing documents return a 404

[jimlambie]

👏