[WIP] segment management using HTTP Endpoints

[nishantmonu51]

Changes for Proposal - #2314
This PR abstracts out zookeeper interactions in the historical and coordinator for segment management.
Changes in Historical Nodes -

1. Extracts segment management from ZKCoordinator into SegmentManager, ZKCoordinator now just listens to the SegmentChangeRequests from Zookeeper and uses SegmentManager to handle those requests
2. Adds new HTTP End points in Historical Resource which uses SegmentManager to process SegmentChangeRequests

Changes in Coordinator -

1. Abstract out LoadQueuePeon which handles load/unload queue and priority for load/unload.
2. ZKLoadQueuePeon uses zookeeper to process SegmentChangeRequests
3. HttpLoadQueuePeon uses the new http end-points for segment management
4. Introduce new config "druid.coordinator.load.peon.type" which can be set to "zk" or "http" (default is http)

TODO:

1. Add unit tests for new endpoints and HttpLoadQueuePeon
2. Add docs

[nishantmonu51]

@guobingkun @drcrallen: Since you are working on similar area, Can you guys do the initial review and provide feedback ?

[drcrallen]

suggest being more descriptive like startStopLock

[drcrallen]

A lot of places seem to be implementing ListenableFutures by hand. Is it reasonable to change the callback stuff to be ListenableFutrue callbacks?

[drcrallen]

parameters should be flipped i think

[himanshug]

@nishantmonu51 are you still planning to work on this? if not, then I can take it up. thanks.

[himanshug]

@nishantmonu51 ping!

[himanshug]

@nishantmonu51 I'm closing this PR and will submit another to support optional segment assignment to http.