Bump all sbt projects (util, ostrich, finagle, twitter-server)

Problem

After looking at the finagle changelog, I discovered that we already had
entries for finagle 6.23.0 and more for the current version.
The previous version bump was then incorrect, the version needs to be
greater than 6.23.0.

Solution

Bump the minor version of finagle, and for consistency I did the same
thing for all the other related projects.
Close the current section of the finagle changelog and name it "6.24.0".

RB_ID=540180