refactor IO layer for cleanliness, integration and performance

After a bit of discussion, the general consensus is that the IO layer should be refactored and more modular and buffer oriented. Well, technically we had the consensus prior to the discussion, but we didn't all trade ideas before then.

Per a meeting on 4 June, the general plan is to:

• Submit all but the current work in progress IO changes that are pending
• Ask Trond to write up his thoughts on the best approach to refactoring libcouchbase (i.e., get to cache coherency) and publish. This can be published anywhere-- this issue is sufficient but it can be a gist, the wiki, etc.
• Ask Sergey and Mark to review and comment in two areas:
  1) will this assist in solving the problems they've seen with NGINX and node.js performance
  2) does this align with their thoughts on the refactoring
• Carry out refactoring work by Sergey, Mark or Trond. Sergey is the component owner and therefore responsible for seeing that it's eventually done, but Mark has done some great work here and Trond may be interested in getting involved. Assuming he won't have time, it'll still be great to have Trond as a reviewer.