Override getThreadPool from HystrixConcurrencyStrategy and delegate to

existing strategy if present. Fixes #2035

This allows clients to send batched up events in the same format as
before (or to continue to send single events). We can switch the
default format to an array in 1.4.x.

Close response when retry on certain status codes

Previously, these were managed directly via archaius, but due to
some combination of changes in boot 1.5.x (likely a tomcat change as
zuul filters were initialzed in a servlet context listener) and Dalston,
The filter was created before archaius could be initialized.

The properties are backwards compatible and are in the
`ZuulProperties.Host` object.

fixes gh-1950

Deprecates RetryableStatusCodeException 

see gh-1950

Let SecurityContextConcurrencyStrategy be installed as the hystrix
concurrency strategy regardless of if feign's hystrix support is installed.

fixes gh-1969

fixes gh-1795

The timeToLive and timeUnit parameters added to configure connection pool.

Fixes gh-1720

fixes gh-1956

fixes gh-1955

Fixes gh-1598

Fixes a race condition introduced in 1.3.0

fixes gh-1933

Add @Component for Feign Hystrix Fallback example in doc

Add support for Wildcard Types in Feign Client.  Fixes gh-1676