enable gRPC keepalive by default

The keepalive is critical for detecting load balancers and other network middlemen that like to silently drop connection data after certain periods of idelness.

Author: benjaminp

src/main/java/com/google/devtools/build/lib/authandtls/AuthAndTLSOptions.java

@@ -116,7 +116,7 @@ public class AuthAndTLSOptions extends OptionsBase {
 
   @Option(
       name = "grpc_keepalive_time",
-      defaultValue = "null",
+      defaultValue = "60s",
       converter = DurationConverter.class,
       documentationCategory = OptionDocumentationCategory.UNCATEGORIZED,
       effectTags = {OptionEffectTag.UNKNOWN},
@@ -126,8 +126,7 @@ public class AuthAndTLSOptions extends OptionsBase {
           sends pings after this much time of no read operations on the connection, but
           only if there is at least one pending gRPC call. Times are treated as second
           granularity; it is an error to set a value less than one second. By default,
-          keep-alive pings are disabled. You should coordinate with the service owner
-          before enabling this setting. For example to set a value of 30 seconds to this
+          keep-alive pings are disabled. For example to set a value of 30 seconds to this
           flag, it should be done as this `--grpc_keepalive_time=30s`.
           """)
   public Duration grpcKeepaliveTime;