* Adds nodes usage API to monitor usages of actions
The nodes usage API has 2 main endpoints
/_nodes/usage and /_nodes/{nodeIds}/usage return the usage statistics
for all nodes and the specified node(s) respectively.
At the moment only one type of usage statistics is available, the REST
actions usage. This records the number of times each REST action class is
called and when the nodes usage api is called will return a map of rest
action class name to long representing the number of times each of the action
classes has been called.
Still to do:
* [x] Create usage service to store usage statistics
* [x] Record usage in REST layer
* [x] Add Transport Actions
* [x] Add REST Actions
* [x] Tests
* [x] Documentation
* Rafactors UsageService so counts are done by the handlers
* Fixing up docs tests
* Adds a name to all rest actions
* Addresses review comments
779fb9a1c0