Deprecate ChannelProvider::configure(), which doesn't do much and
is incompatible with the idea of shared context.
A lot of down-stream mess related to the confused relationship
between InternalClientContextImpl and InternalClientContextImpl::ChannelProviderImpl.
This is changed to compose the provider within the context
and use a nested shared_ptr so that references to the provider
are really references to the context.
This brings the ownership semantic in line with what the API
suggests, and what other providers implement.
Michael Davidsaver
c0ee432598