Reduce heap allocations in BLASTBufferQueue.

* Replace std::unordered_map members with ftl::SmallMap with empirically
  determined sizes.
* Eliminate creating an unnecessary std::vector

Test: unit, manual
Bug: 285377983
Flag: EXEMPT refactoring
Change-Id: I20402bf58d6619573cb1ff85be5092447f841b8a
3 files changed