API: Proposal for additional IO buffer and body template API.

[SolidWallOfCode]

This is not intended to be merge immediately because it describes API that has not yet been written. Comments should be about the API itself.

The new functions are
TSIOBufferReaderRead
TSIOBUfferReaderIterate
TSHttpTxnErrorBodyBufferSet
TSHttpTxnApplyLogFormat

The other documentation is of API that is already present but undocumented. Feel free to comment in that regard.

[SolidWallOfCode]

I added the new function TSIOBufferReaderIsAvailAtLeast because this has proved handy in the core and (since it's an existing method) this will be easy to implement. I also cleared up how blocks are consumed by TSIOBufferReaderIterate.

[jpeach]

TSHttpTxnApplyLogFormat

• I think the name could be clearer, maybe TSHttpTxnFormatLogTags() or TSHttpFormatTags().
• It seems reasonable to format to an IO buffer, but if you think
  you would like to also be able to format to a pre-allocated buffer
  (pointer + length), we should choose the right name now so that
  later they will name sense.

TSHttpTxnErrorBodyBufferSet

• The description text doesn't seem to match.
• Since this is clearly intended to be paired with the log formatter,
  it would be helpful to explicitly join the dots in the description.
• I don't think you need the MIME type argument since it is easy
  to set the Content-Type header using existing API. If you think
  the existing API is too troublesome, maybe a secondary helper API
  that can set a named header?

You should go ahead and commit the TSIOBufferReaderAlloc man page,
but call it TSIOBufferReader since it is documenting the whole API
family (which makes sense).

[SolidWallOfCode]

See https://issues.apache.org/jira/browse/TS-2375

[bryancall]

@SolidWallOfCode said he would work on it this quarter.

[zwoop]

@SolidWallOfCode You going to land this in this incarnation? Or another PR?