README

Organization of the meta crates

We split the meta module into smaller crates in order to speed up compilation.

• meta/node is the final meta node server
• meta/service is tonic grpc service implementations. We may further split this into parallel sub-crates.
• The remaining part meta/src is the implementation details imported by services. In the future, we can also try to re-organize this into smaller units.

Refer to #12924 for more details.