Move the pull request guide to the developer guide

Motivation:

CONTRIBUTING.md is useful only because it lets Github show a user the
link to it so the user can check what information we need before
submitting a bug report.  However, Github does not do the same for a
pull request submission form, and thus there's no reason to keep the
information about how to submit a good pull request in CONTRIBUTING.md.

Modification:

Replace the section about issuing a pull request with the link to the
official developer guide.

Result:

CONTRIBUTING.md is easier to maintain.
Overall clean-up of ipfilter package

Motivation:

Recently merged ipfilter package has the following problems:
* AbstractIpFilterHandler could be improved to support any SocketAddress types rather than only InetSocketAddress.
* AbstractIpFilterHandler can be removed immediately after decision is made rather than keeping the outcome of the decision as an attribute.
* AbstractIpFilterHandler doesn't have a hook for the accepted addresses.
* The hook method (reject()) needs to be named in line with other handler methods (i.e. channelRejected())
* IpFilterRuleHandler should allow accepting zero rules - it's particularly useful for machine-configured setup (i.e. specifying zero rules disables ipfilter).
* IpFilterRuleType.ALLOW/DENY should be ACCEPT/REJECT for consistency.

Modifications:

* AbstractIpFilterHandler has been renamed to AbstractRemoteAddressFilter and now uses type parameter.
* Added channelAccepted() and renamed reject() to channelRejected()
* Added ChannelHandlerContext as a parameter of accept() so that accept() can add a listener to the closeFuture() of the channel. This way, UniqueIpFilter continue working even if we remove the filtering handler early.
* Various renames
  * IpFilterRuleHandler -> RuleBasedIpFilter
  * UniqueIpFilterHandler -> UniqueIpFilter

Result:

* Much cleaner API with more extensibility
Do not use finally to propagate events in AbstractRemoteAddressFilter

Motivation:

We don't really need to propagate an event when handling the event fails.

Modifications:

Do not use finally block in AbstractRemoteAddressFilter

Result:

AbstractRemoteaddressFilter does not forward an event in case of failure.
Add capacity limit to Recycler / Optimize when assertion is off

Motivation:

- As reported recently [1], Recycler's thread-local object pool has unbounded capacity which is a potential problem.
- It accesses a hash table on each push and pop for debugging purposes.  We don't really need it besides debugging Netty itself.

Modifications:

- Introduced the maxCapacity constructor parameter to Recycler.  The default default maxCapacity is retrieved from the system property whose default is 256K, which should be plenty for most cases.
- Recycler.Stack.map is now created and accessed only when assertion is enabled for Recycler.

Result:

- Recycler does not grow infinitely anymore.
- If assertion is disabled, Recycler should be much faster.

[1] https://github.com/netty/netty/issues/1841
Remove condition in ChannelHandlerAdapter.isSharable() by caching the result of the annotation lookup.

Motivation:
Remove the synchronization bottleneck and so speed up things

Modifications:
Introduce a ThreadLocal cache that holds mappings between classes of ChannelHandlerAdapater implementations and the result of checking if the @Sharable annotation is present.
This way we only will need to do the real check one time and server the other calls via the cache. A ThreadLocal and WeakHashMap combo is used to implement the cache
as this way we can minimize the conditions while still be sure we not leak class instances in containers.

Result:
Less conditions during adding ChannelHandlerAdapter to the ChannelPipeline
Fix checkstyle errors introduced by f0d1bbd63ec910b9c5bccc925bdf0b0f55db1f9c