Eliminate memory copy in ByteToMessageDecoder whenever possible

Motivation:

Currently when there are bytes left in the cumulation buffer we do a byte copy to produce the input buffer for the decode method. This can put quite some overhead on the impl.

Modification:

- Use a CompositeByteBuf to eliminate the byte copy.
- Allow to specify if a CompositeBytebug should be used or not as some handlers can only act on one ByteBuffer in an efficient way (like SslHandler :( ).

Result:

Performance improvement as shown in the following benchmark.

Without this patch:
[xxx@xxx ~]$ ./wrk-benchmark
Running 5m test @ http://xxx:8080/plaintext
  16 threads and 256 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    20.19ms   38.34ms   1.02s    98.70%
    Req/Sec   241.10k    26.50k  303.45k    93.46%
  1153994119 requests in 5.00m, 155.84GB read
Requests/sec: 3846702.44
Transfer/sec:    531.93MB

With the patch:
[xxx@xxx ~]$ ./wrk-benchmark
Running 5m test @ http://xxx:8080/plaintext
  16 threads and 256 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    17.34ms   27.14ms 877.62ms   98.26%
    Req/Sec   252.55k    23.77k  329.50k    87.71%
  1209772221 requests in 5.00m, 163.37GB read
Requests/sec: 4032584.22
Transfer/sec:    557.64MB
Cruft?