servenv: add new keepalive flag to grpc server

[derekperkins]

This PR adds a single grpc configuration flag for keepalive enforcement policy: PermitWithoutStream:. It defaults to false, the same as previous behavior. The purpose is to prevent potential zombie connections from vtgate to vttablet on intermittently active streams. We're already setting this to be true by default on the client side here:

vitess/go/vt/grpcclient/client.go

Lines 68 to 75 in 2af0d63

	kp := keepalive.ClientParameters{
	// After a duration of this time if the client doesn't see any activity it pings the server to see if the transport is still alive.
	Time: *keepaliveTime,
	// After having pinged for keepalive check, the client waits for a duration of Timeout and if no activity is seen even after that
	// the connection is closed. (This will eagerly fail inflight grpc requests even if they don't have timeouts.)
	Timeout: *keepaliveTimeout,
	PermitWithoutStream: true,
	}

That setting doesn't take effect if it isn't also enabled server side.

// If true, server allows keepalive pings even when there are no active
// streams(RPCs). If false, and client sends ping when there are no active
// streams, server will send GOAWAY and close the connection.
https://godoc.org/google.golang.org/grpc/keepalive#EnforcementPolicy

[sougou]

That is a long flag name :)