grpc-js: Fix keepalive ping timing after inactivity

[murgatroid99]

This fixes the implementation of the keepalive spec, specifically the following paragraphs:

Since keepalive is not occurring on HTTP/2 connections without any streams, there will be a higher chance of failure for new RPCs following a long period of inactivity. To reduce the tail latency for these RPCs, it is important to not reset the keepalive time when a connection becomes active; if a new stream is created and there has been greater than 'keepalive time' since the last read byte, then a keepalive PING should be sent (ideally before the HEADERS frame). Doing so detects the broken connection with a latency of keepalive timeout instead of keepalive time + timeout.

keepalive time is ideally measured from the time of the last byte read. However, simplistic implementations may choose to measure from the time of the last keepalive PING ack (a.k.a., polling). Such implementations should take extra precautions to avoid issues due to latency added by outbound buffers, such as limiting the outbound buffer size and using a larger keepalive timeout. Implementations must not measure from the last keepalive PING sent, to avoid triggering abuse detection on servers that are configured with settings that exactly match behavior of the client.