finagle-core: RandomSocket cleanup

RB_ID=548674
finagle-core: c.t.f.builder.Server implements ListeningServer

Problem

The current ServerBuilder interface returns its own Server trait. This
trait is lacking features like announcing and Await-ability. Most
documentation and examples are assuming that users are dealing with a
ListeningServer.

Solution

Turn the Server trait returned by ServerBuilder into a ListeningServer.

Result

Users can use features from ListeningServer when using the ServerBuilder
interface. More docs for the finagle 6 interface apply to ServerBuilder.

RB_ID=547434
TBR=true
finagle-core: Retry forever on WriteExceptions

Problem

WriteExceptions shouldn't have to count against your retry budget,
and should be retried automatically.

Solution

Retrying on WriteExceptions eagerly breaks down without failfast
turned on, and we decide whether to failfast in two places, the
codec and the flag.  I added a RetryingFilter to the stack which
retries 100 times on WriteException, and changed the StackClient
to remove it if it doesn't see a FailFastFactory in the Stack.

Result

We will retry on every finagle "logical" request 100 times on
WriteException.  The RetryingFilters and RetryingPolicies that
people apply should in practice no longer need to account for
WriteExceptions.

RB_ID=540478
finagle-core: Let FailFast be configured for Stack based clients

Problem

It is difficult for Stack-based clients to disable fail fast
which happens to be enabled by default.

Solution

Introduce `c.t.f.service.FailFastFactory.FailFast` which allows
Stack-based clients to configure this behavior.

RB_ID=548254
Reinstate casts that are necessary for Scala 2.11 compatibility

RB_ID=549962
Use Travis CI's new container-based infrastructure

Problem

Travis CI just announced a new container-based infrastructure that provides
faster builds, etc., and can be enabled by disabling sudo in the Travis config.

Solution

We don't need sudo for any of these projects, so we can disable it. I've also
fixed a few minor issues with versions in the Travis configuration for scrooge,
finagle, and twitter-server.

RB_ID=547354