Add the new docs "batch write" endpoint

[EricBorczuk]

See stargate/stargate#1043 for technical details

An endpoint was added that allows a user to send a single request to create multiple documents at once. The format of such a request is:

POST /api/v2/namespaces/:namespace/collections/:collection/batch

[
    {"data": "doc1", "some":"info"},
    {"data": "doc2", "some":"info"},
    {"data": "doc3", "some":"info"},
    {"data": "doc4", "some":"info"},
]

This will create 4 new documents with ID's that are generated UUID's. The response:

202 Accepted

{ "documentIds": [...] }

Optionally, an id-path query parameter can be supplied, which allows taking a particular path within each document and using that as an ID. as an example:

POST /api/v2/namespaces/:namespace/collections/:collection/batch?id-path=data.[0]

[
    {"data": ["doc1"], "some":"info"},
    {"data": ["doc2"], "some":"info"},
    {"data": ["doc3"], "some":"info"},
    {"data": ["doc4"], "some":"info"},
]

This will create 4 new documents with ID's that come from the documents themselves. The response:

202 Accepted

{ "documentIds": ["doc1", "doc2", "doc3", "doc4"] }

[polandll]

#217