Add io.netty.noJdkZlibEncoder system property

Related issue: #2821

Motivation:

There's no way for a user to change the default ZlibEncoder
implementation.

Modifications:

Add a new system property 'io.netty.noJdkZlibEncoder'.
Use JZlib-based encoder if windowBits or memoryLevel is different from
the JDK default.

Result:

A user can tell HttpContentCompressor not to use JDK ZlibEncoder even if
the current Java version is 7+.