Dynamic resize

If a buffer is inactive for too long, its size gets shrunk to 10 instead
of 1024, and gets re-established when reconnecting.
Fix buffer resizing bug

When a buffer gets resized and needs to drop data, the data is dropped,
but the maximal value of the buffer is never brought down. This patch
makes it so the function works as expected.

This fix also removes all the unused conversion functions that were used
in a past deploy.
Introduce ENV variable for tcp drain buffer size
Add tests for TCP drain resizing
Force shrinking on never-connected drains

Only a successful connection will grow the drain size.
shrinking of HTTP drains

Inactive drains (disconnected for too long) get shrunk down, and resized
when going back online.
Merge pull request #62 from heroku/dynamic_buf_size

Dynamic buf size