Bitcask merge visibility [JIRA: RIAK-2412]

[slfritchie]

It would be nice to have better visibility into bitcask merge activity.

1. Log events when merge starts?
2. A public ETS table (very low update frequency, to avoid worries about performance impact) that tracks which bitcask instances are merging (and perhaps also list of input files)?

[bsparrow435]

+1 I generally use {log_needs_merge, true} to get a start of merge log and any information we can provide on the status or progress of a merge would be fantastic. Data like how long since last merge, merge progress, etc would be a huge win.

[evanmcc]

How does this sound: an info message when the merge starts, and cleaning up the post merge message to be just the vnode index and a list of file ids so that it doesn't take up so much space in the logs?

I'd also like to make that change to needs_merge logging as well.

[bashopatricia]

Not planned / scheduled