Push Txns & Resolve Intents on Conflicts

Automatically push transactions on write intent failures and resolve
conflicting write intents. This is done at the level of
Store.ExecuteCmd on command failure. Write/write conflicts are
pushed and resolved but then must still return the write intent
error to the client so the client can retry with a new client command
ID, ensuring idempotence. This way, all original requests (the one
which hit the conflict) will always return a write intent error via
the response cache. Follow-on requests with the new client command
ID can succeed, and with new ClientCmdID, will always succeed on
retries. Read/write conflicts are automatically retried instead of
returning the write intent error, because they don't have the same
idempotency issues with client command ID.

Reorg'd a bit of code between store and range to move range-specific
checks into range (addition of Range.AddCmd method). Also, added an
ability to submit commands to Raft without having to always wait for
completion. This is used to "fire and forget" resolve intent cmds.
Making sure we cleanup entries pending in the "read queue" is slightly
tricky in Range.addReadWriteCmd.

Moved writeIntentError and writeTooOldError from mvcc.go to the proto/
subdirectory as they're communicated across RPCs from server to clients.
Add the conflicting Key and also a Resolved flag to WriteIntentError.
This allows the client to decide whether to backoff (if resolve failed)
or retry immediately.