* grpcapi: added GPRC API for the repository server
* repo: added transparent retries to GRPC repository client
Normally GRPC reconnects automatically, which can survive server
restarts (minus transient errors).
In our case we're establishing a stream which will be broken and
needs to be restarted after io.EOF is detected.
It safe to do transparent retries for read-only (repo.Repository),
but not safe for write sessions (repo.RepositoryWriter), because the
session may re-connect to different server that won't have the buffered
content write available in memory.
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