WIP: early share work towards experimental streaming support

[jcorbin]

Much stil TODO:

• break out and regression test the partial read fix to fragmentingReader
• break out and finish the new ErrorHandlerFunc convenience
• refactor the withArg2/withArg23 helpers added in client.go to be instead
  methods on tchannel.OutboundCallResponse
• refactor the withArg2/withArg23 helpers added in server.go to be instead
  methods on tchannel.InboundCall
• implement response coalescing under the server watch handler
• implement error/cancellation support in the server watch handler
• implement finer grained streaming timeout support: the context timeout
  should apply to only the first frame of a stream, with a separate longer
  timeout for the lifetime of the entire streamed request and/or response
  expressed as a request header (and then maybe modified by the initial
  response frame?)

cc @prashantv @nomis52

[prashantv]

Closing stale PR against master, filed #466 to fix this issue (and have a branch with a slightly different fix). However, since it doesn't affect any users it's not a high priority issue right now.