Improve EOM aggregator performance

The decoding performance of DelimiterBasedFrameDecoder, and hence the performance of NetconfEOMAggregator extending DelimiterBasedFrameDecoder becomes the bottleneck of NETCONF channel pipeline when small increments of a long frame are processed. For example, fetching YANG schema files (total 22 MB) from Juniper MX2020 takes about 16 minutes.

The main reason for this poor performance is because delimiter search starts over from the beginning of the frame (readerIndex) for each decode call in the indexOf method.