Port neo4j-com to netty4.

This moves neo4j-com (and therefore neo4j-ha and neo4j-backup) to netty4.
Because netty3 and netty4 have different packages and different artifact names,
neo4j-cluster can keep using netty3 without conflict.

Netty4 ships with fairly advanced buffer pooling, cutting GC load significantly
compared to netty3. In this commit, buffer pooling is disabled, meaning neo4j-com
behaves exactly like it did with netty3. This is a minimum set of changes.

In a follow up commit we can then go through the work of undoing our own pooling,
replacing it with the more comprehensive pooling offered in netty4.

Likewise for neo4j-cluster, it can be ported at leisure, independently from this commit.