Avoid copying client interceptor contexts

[glbrntt]

Motivation:

The client interceptor pipeline holds an array of interceptor contexts.
Each context is a struct and also holds a reference to the pipeline.
Each time the context asks the pipeline for the next inbound/outbound
context we incur a copy of the context which is surprisingly costly due
to retain and release traffic for the pipeline ref as well as the cost
of initializing the unspecialized contexts.

Modifications:

• interceptor contexts now ask the pipeline to perform an operation for
  them rather than asking for a context on which to perform the
  operation, this avoids copying contexts
• as the pipeline is now responsible for performing the operations a
  further optimisation is opened up: head and tail interceptors and
  contexts are no longer required, the same action can be done directly
  by the pipeline

Result:

A 4.8% reduction in instructions in the unary_10k_small_requests
benchmark.