L116: C++-Core: Loosen behavior of GRPC_ARG_HTTP2_MAX_PINGS_WITHOUT_DATA

[yas...@google.com]

Hey all, 

https://github.com/grpc/proposal/pull/429 is a proposal to modify the behavior GRPC_ARG_HTTP2_MAX_PINGS_WITHOUT_DATA to throttle pings to a frequency of 1 minute instead of completely blocking pings when too many pings have been sent without data/header frames being sent.

Comments are welcome!