Add log service support

[subfuzion]

Add an rpc service for querying logs stored in elasticsearch. Provide support for queries with various filter criteria. Provide a cli logs command for querying the service.

Our concept of log collation corresponds to Docker Cloud's log multiplexing and is very similar to the work that @generalhenry worked on with amp-log-service. @freignat91 will be working on the log producer side and adding the cli interface for it (see #15). See Docker Cloud's docs for comparison (we intend to provide range queries, filtering, etc).

[subfuzion]

@bertrand-quenin You're getting very close with the basic log service. Please use the cmd/amp/cli package for any CLI-side command helpers; the api/client package shouldn't be aware of anything that has to do with parsing command line options (it should simply take parameters that it needs to do its work). Also, you can move the Logs function out of api/client/amp.go and put it in its own command helper, like api/client/logs.go.

Go ahead and start adding the log stream support as well that we've been discussing in the Observability document on Google Drive. The goal is to be able to start a service with a few replicas and be able to get access to a multiplexed log stream from the CLI. You should be able to automate testing based on capturing one of the existing swarm services.

Please don't forget to remove the code we don't need right now per this comment.

[subfuzion]

@bertrand-quenin Thanks for a nice job. I'm going to close this issue now. We can open new issues for specific enhancements, etc.