allow to enable/disable dead letter feature. Graylog2/graylog2-web-interface#653
bump to 0.20.1
add recvbuffersize to input config dialogs

to aid in tuning the receive buffer sizes for inputs, we now expose the settings in the relevant inputs.
enabled for both tcp and udp of the syslog, gelf and raw inputs so far.
the default value is still 1MB and configurable in the graylog2.conf

fixes #454
bump internal server version to 0.20.2-snapshot
merge 0.20