Fast RFC2616 Date parser

This PR introduces the following changes:
* A very fast [RFC2616 date](http://tools.ietf.org/html/rfc2616#section-3.3) parser that replaces the old `SimpleDateFormat`+`ThreadLocal` based implementation in `AsyncHttpProviderUtils.parseExpireField`. It uses a `TimeConverter` for converting date parts into a time. Default implementation is `Calendar` based, but one can pass a custom implementation in the config, such as a JodaTime or Java8 DateTime based one. Currently, only the Netty provider can use this user-defined TimeConverter because the Grizzly one uses static methods and singletons where I could not pass the config. Benchmark results:
  * **old impl: 1729 ops/s**
  * **new impl w/ Calendar (default): 6608 ops/s**
  * **new impl w/ JodaTime (custom) : 8409 ops/s**
  * **new impl w/ ThreeTenBP (custom) : 8492 ops/s**
* It breaks the old Cookie behavior where `Expires` attribute was turned into a `Max-Age`. This behavior made testing a mess (Cookie.maxAge depended on currentTimeMillis). I separated both into dedicated attributes. Now `AsyncHttpProviderUtils` has a `secondsFromNow` for computing a maxAge from a expires value.
* It no longer supports multiple cookies inside the same `Set-Cookie` header. RFC6265 doesn't support this, as well as modern browsers that only pick the first one.
* It fixes a bug regarding `Set-Cookie2` header handling: it has precedence over regular `Set-Cookie`.