(RFC:https://github.com/grpc/proposal/pull/3). Add necessary
microbenchmarks to show reduced writes/iter for short streams. Add
necessary end2end test to test out newly added apis. Filter those tests
using *WithCoalescingApi*.

This reverts commit 5e01e2ac.

GRPC_INITIAL_METADATA_WAIT_FOR_READY.
Also add a flag to indicate whether wait_for_ready was explicitly set by
the application.

In some cases, the prototype argument order was different
than the declaration order.

Based on a patch from Sam Leffler.