Prepare next development cycle: 0.21.0-rc2-SNAPSHOT
Remove obsolete local copy of syslog4j
Increase default output_batch_size to 1000, fixes #705
Update documentation links to point to the new graylog2.org page.
Revert "Increase default output_batch_size to 1000"

This reverts commit 574a2e4f78aea6bc9f0e1673e005acb26697eb76 until the problems
described in #705 have been investigated.
Add config parameters to configure Netty HttpRequestDecoder, fixes #707
Clean up org.graylog2.plugin.Message
Handle edge case in RegexExtractor, fixes #708
Change argument for Message#setStreams back to List<Stream>

This was b0rked in commit ab4e8caa8c8256b5236fc63a190c7aaa63588d70.
Upgrade to Metrics 3.1.0
Migrate to Log4j 2.x

The primary API being used for logging is still SLF4J but the underlying
logging framework has been migrated from Log4j 1.x to Log4j 2.x. All
transitive dependencies to Log4j 1.x have been excluded and the compatibility
bridge of Log4j 2.x (log4j-1.2-api) has been pulled in instead of the SLF4J
bridge (log4j-over-slf4j).

All relevant usages of the Log4j 1.x API have been replaced by the corresponding
Log4j 2.x implementations and the default configurations have been updated to
work with Log4j 2.x (log4j2.xml and log4j2-test.xml).

The logging framework has been configured to use the new asynchronous loggers
of Log4j 2.x (see https://logging.apache.org/log4j/2.x/manual/async.html and
org.graylog2.shared.NodeRunner) which is also using the Disruptor and should
yield some interesting performance improvements for the internal logging of
Graylog2.