Currently, we're doing "sync" in databroker server. If we're going to
support multiple databroker servers instance, this mechanism won't work.
This commit moves the "sync" to storage backend, by adding new Watch
method. The Watch method will return a channel for the caller. Everytime
something happens inside the storage, we notify the caller by sending a
message to this channel.
* pkg: add storage package
Which contains storage.Backend interface to initial support for multiple
backend storage.
* pkg/storage: add inmemory storage
* internal/databroker: use storage.Backend interface
Instead of implementing multiple databroker server implementation for
each kind of storage backend, we use only one databroker server
implementation, which is supported multiple storage backends, which
satisfy storage.Backend interface.
