db_test assertion - changing dynamically write buffer size not aligned with pending memtable creation

[ayulas]

in db test changing dynamically write buffer size and it is not aligned to the fact we have an active memtable and a pending memtable...
so the check of the L0 files (meainng the memtable that have done flush) depending the increase/decrease write buffer size is incorrect cause the pending memtable is created with the previous write buffer size ...

[ayulas]

Found a way to fix it. i tested and it was solved.
i will transfer for review

the fix is in the code.... not in the test.
when i create the pending memtable i dont activate it with allocator resources that cause the issue (part of the flush threshold but it shouldn't , cause it doent in the game yet) when it turns active i set the current cf values to the activate memtable so if the write buffer size was changed after the pending memtable was created , it will get the correct value as part of activation and the test will get what it want.

note that the memtable object creation is not part of the flush threshold - the bucket shouldnt be calculate as a write buffer. so it doesnt use the arena. (it is like that today - just mention it to explain that its ok to set the arena parameters after the memtable was created but wasnt activated yet)

[isaac-io]

Found in #14 (comment).

[isaac-io]

Irrelevant since #25 was reverted.