maintain affinity of next_channel whenever possible

Keep code that is common between both async client versions in a new async superclass and only use unary/streaming-specific material in the subclass.

Testing asan configuration with the latest clang compiler fails due to
the warning `-Winconsistent-missing-override`. This patch fixes it.

This reverts commit 45b0bc4b.

This revert is being done because the compilers on Travis don't
understand the C++11 template/using syntax.