Choose default grpc_subchannel_count for publisher

[suzmue]

Go by default has 4 grpc connections, we should consider using the same as a default for the publisher to match performance.

[suzmue]

.Net uses # of CPUs as the default value

[coryan]

.Net uses # of CPUs as the default value

Large VMs can have 300+ cores, and the default limit for file descriptor count is (as I recall, check this number) only 1024. If an application creates several Pub/Sub clients or several clients for different services, we could consume all the file descriptors rather quickly.