Log tailing fixes and polish

Fixes the bug where you could never scroll up when in "tailing mode".

Also, adds some polish:

  - Shows "fetching more lines.." at the top and bottom if there is more to fetch (need to put a spinner in them)
  - Don't clear out existing content when you click "go to top/bottom" unless necessary.
  - Exponentially grow the request size as you frequently request more lines (to a limit)

Note, I'd still like to test a little more before merging, but wanted to get something up. /cc @wsorenson