Allow to disable only heap or direct buffer pool

- Fixes #1315

If a user specifies the arena size of 0, the pool is now disabled
instead of raising an IllegalArgumentException. Using this, you can
disable only heap or direct buffer pool easily. Once disabled,
PooledByteBufAllocator will delegate the allocation request to
UnpooledByteBufAllocator.
Fix a bug where HttpContentEncoder generates an empty chunk even if it's not the last chunk

- Fixes #1312
- Added more test cases to ensure the fix
update to barchart-udt 2.2.6
* which fix https://github.com/barchart/barchart-udt/issues/48