client: Ensure active/healthy connection prior to solve

Provide a WaitForReady client method that performs a preflight request and specifies grpc.WaitForReady(true) to ensure that the grpc.ClientConn has established the underlying connection and that it can be considered available.

Performing this request prior to solves makes the client more robust in environments where the server is behind a proxy or part of a service mesh (e.g. Istio/Envoy). In these environments, connections may be prematurely closed prior to any client requests due to circuit breaking on max connections.

For the moment, this incurs a redundant request to ListWorkers which seemed to be the more backwards compatible request to make as a preflight; Info is not available in older versions. If buildkitd ever implements the grpc.health.v1.Health directly on its server endpoint, a grpc.health.v1.Health/Check may make more sense.

Signed-off-by: Dan Duvall dduvall@wikimedia.org