Allow starting and stopping Fusion Checkpointing at vbucket level

First, we expose a initial_fusion_checkpointing_state param in the EP config (bucket level) that is used at VBucket creation as default initial checkpointing state for the new VBucket.

Then, ns_server needs to to be able to start and stop Fusion Checkpointing at need. That is a vbucket-level state.
Alternatives:

• Introduce a new MCBP command
• Implement via SetParam vbucket_param (existing API). This is the preference at the time of writing

We extend the existing SetParam API for supporting the follwoing:

SetParam("vbucket_param", <vbid>, "fusion_checkpointing_enabled", <"true"|"false">)