Get rid of Netty's DelimiterBasedFrameDecoder

Get rid of Netty's DelimiterBasedFrameDecoder

Netty's DelimiterBasedFrameDecoder is not really effective when dealing
with huge NETCONF messages and reading them in just really small parts
at a time.

DelimiterBasedFrameDecoder always searches whole input ByteBuf for
delimiter when new input bytes are available. It performs the search
also on bytes already checked in previous tries. Obviously, this can be
really ineffective when reading large messages by very small parts at
a time.

Replace Netty's DelimiterBasedFrameDecoder by our own frame detection
implementation. The implementation remembers already checked part of
the input ByteBuf and searches the delimiter just in a new bytes
available.

JIRA: NETCONF-889
Change-Id: If5e78f4373d767f4cf465024313eeac873c1621d
Signed-off-by: Jakub Morvay <jmorvay@frinx.io>
Signed-off-by: Maros Marsalek <mmarsalek@frinx.io>
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>