New bucket setting for XDCR/mobile co-existence

As part of XDCR-mobile co-existence, mobile and XDCR will maintain HLV (Hybrid Logical Vector) across clusters. To do that, we need two settings at bucket level.

First setting is to enable the feature:

Key: enableCrossClusterVersioning

Input Behavior:

• It is a boolean value that can be specified via REST, and toggled via UI.
• When it is set to true, the system internally should record the max_cas for the VBs corresponding to when the action is set.
• When it is set to false, the system internally should discard the recorded max_cas.

Output Behavior

• the setting value as well as the corresponding max_cas for the VBs should be accessible through a REST command.

Streaming API Behavior:

• Any output changes to the setting should be available through stream API /pools/default/bucketsStreaming

So XDCR does not have to constantly poll ns_server to catch any changes.

It should also be exposed via /pools/default/buckets/bucketname so Sync Gateway can access it.

Second setting is for the HLV maintenance:

Key: versionPruningWindowHrs

• Minimum: 1 day
• Maximum: MAXINT

The setting change should be available through stream API 

/pools/default/bucketsStreaming

So XDCR does not have to constantly poll ns_server for the value.

It should also be exposed via 

/pools/default/buckets/bucketname

So Sync Gateway can access it.

See "Settings" section in this document for more information: https://docs.google.com/document/d/1S0WStgbqRGlmwbdNyHn_9qqEqvshPv9WyG8skauJDEc/edit?usp=sharing