Make SizeClasses as composition with PoolArena instead of inheritance to reduce memory usage

[laosijikaichele]

Motivation:

For one PooledByteBufAllocator instance, the array PooledByteBufAllocator.heapArenas[] only needs one SizeClasses instance, so does the PooledByteBufAllocator.directArenas[], this will reduce memory usage.

Take the PooledByteBufAllocator.DEFAULT as an example, on my laptop, after PooledByteBufAllocator.DEFAULT initialized, there are heapArenas[16] and directArenas[16] arrays created, every element of the arrays created following int[] array by SizeClasses's constructor:

int[] pageIdx2sizeTab; // Length is 32 on my laptop
int[] sizeIdx2sizeTab; // Length is 68 on my laptop
int[] size2idxTab; // Length is 256 on my laptop

After PooledByteBufAllocator.DEFAULT initialized, there are 32 int[] pageIdx2sizeTab, 32 int[] sizeIdx2sizeTab, and 32 int[] size2idxTab arrays created.

This is not necessary, because the PooledByteBufAllocator.heapArenas[] can share one SizeClasses instance, so does the PooledByteBufAllocator.directArenas[].

After the PR change, the PooledByteBufAllocator.DEFAULT only needs 2 int[] pageIdx2sizeTab, 2 int[] sizeIdx2sizeTab and 2 int[] size2idxTab arrays.

It reduced at least 42720 bytes memory usage when use PooledByteBufAllocator.DEFAULT:
4 * (30 * 32 + 30 * 68 + 30 * 256) = 42720 bytes.

So we make SizeClasses as composition with PoolArena instead of inheritance.
And it still reserves the ability to create SizeClasses per PoolArena for possible different implementation in the future.

Modification:

Make SizeClasses as composition with PoolArena instead of inheritance.

Result:

Reduced memory footprint.

[normanmaurer]

@laosijikaichele PTAL and fix check style

[laosijikaichele]

@normanmaurer done.

[chrisvest]

No need to forward port this because main does a similar thing but differently.