DRILL-4728: Add support for new metadata fetch APIs

+ Protobuf messages
   - GetCatalogsReq -> GetCatalogsResp
   - GetSchemasReq -> GetSchemasResp
   - GetTablesReq -> GetTablesResp
   - GetColumnsReq -> GetColumnsResp

+ Java Drill client changes

+ Server side changes to handle the metadata API calls
  - Provide a self contained `Runnable` implementation for each metadata API
    that process the requests and sends the response to client
  - In `UserWorker` override the `handle` method that takes the `ResponseSender` and
    send the response from the `handle` method instead of returning it.
  - Add a method for each new API to UserWorker to submit the metadata work.
  - Add a method `addNewWork(Runnable runnable)` to `WorkerBee` to submit a generic
    `Runnable` to `ExecutorService`.
  - Move out couple of methods from `QueryContext` into a separate interface
    `SchemaConfigInfoProvider` to enable instantiating Schema trees without the
    full `QueryContext`

+ New protobuf messages increased the `jdbc-all.jar` size. Up the limit to 21MB.

this closes #527

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