grpclb needs to connect the second resolved balancer address when the first balancer disconnects.
If grpclb gets 2 resolved addresses: balancer1 and balancer2. When balancer1 disconnects, grpclb should automatically start to use balancer2.
This will prevent the incoming RPCs' metadata from appearing in outgoing RPCs
unless it is explicitly copied, e.g.:
incomingMD, ok := metadata.FromContext(ctx)
if ok {
ctx = metadata.NewContext(ctx, incomingMD)
}
Fixes#1148
This modifies the WithBlock behavior somewhat to block until there is at least
one valid connection. Previously, each connection would be made serially until
all had completed successfully, with any errors returned to the caller. Errors
are now only returned due to connecting to a backend if a balancer is not used,
or if there is an error starting the balancer itself.
Fixes#976
