Keep-alive - half the expected rate

[vassilvk]

When using the -keepalive-time option, grpcurl sends keepalive pings to the server at half the expected rate.

Steps to reproduce

• Set up a gRPC service which listens, for example, on endpoint 1.2.3.4:123
• Run grpcurl with a 60 seconds keepalive-time ping against an API which takes say 5 min to complete:

./grpcurl -keepalive-time 60 1.2.3.4:123 my.Service/ExtremelySlowAPI

• Run tcpdump to monitor packets being sent from grpcurl to the service:

tcpdump -i eth0 dst 1.2.3.4

Expected

You should see keepalive ping packets sent on the wire every 60 seconds.

Observed

tcpdump reports 2 keepalive ping packets sent at once, every 120 seconds.

[jhump]

This should be fixed as of v1.5.0 (as part of updating grpc-go dependency in #136).