[split]  Introduce DefaultTracer that load available Tracers via LoadService + Fix equality with NullStatsReceiver

RB_ID=128930
[split]  Update sbt project definition for util/finagle

RB_ID=134416
[split]  Improvements to util-zk NativeConnector - Fix an assertion error in the util-zk NativeConnector.  releasePromise had been fired when we were entering Connection.apply -- fix this by calling release/apply within the serialized block in - Explicitly handle connection timeouts (and release the Connection in these case).

RB_ID=82495
[split]  bump util to 6.2.6-SNAPSHOT - bump util to 6.2.6-SNAPSHOT - release 6.2.5 of util - update changelog

RB_ID=136016
[split]  time conversions: special-case Duration.Zero to avoid allocation

RB_ID=136102
[split]  util: deprecate Future#apply, get.

RB_ID=136045
[split]  Future.get: include fatal exceptions

3d7103997 accidentally introduced a NonFatal filter.

RB_ID=137058
[split]  util-core: flag a bug with U64 truncation

RB_ID=137120
[split]  release 6.3.0 of util

 - bump util to 6.3.1-SNAPSHOT
 - release 6.3.0 of util
 - bump util minor
 - update changelog
 - csl release: jump to the right dir on large bumps

RB_ID=137918
TBR=true
[split]  deprecate com.twitter.util.concurrent.Concurrent{Pool, MultiMap}

RB_ID=138039
[split]  finagle: bump sbt to 2.3.0

- finagle: bump sbt to 2.3.0
- util: bump sbt to 2.3.0

RB_ID=138034