tsan: increase dense slab alloc capacity

We've got a user report about heap block allocator overflow.
Bump the L1 capacity of all dense slab allocators to maximum
and be careful to not page the whole L1 array in from .bss.
If OS uses huge pages, this still may cause a limited RSS increase
due to boundary huge pages, but avoiding that looks hard.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D101161