Bug 1728069

.The default BlueStore and BlueFS allocator is now `bitmap` Previously, the default allocator for BlueStore and BlueFS was the `stupid` allocator. This allocator spreads allocations over the entire device because it allocates the first extent it finds that is large enough, starting from the last place it allocated. The `stupid` allocator tracks each extent in a separate B-tree, so the amount of memory used depends on the number of extents. This behavior causes more fragmentation and requires more memory to track free space. With this update, the default allocator has been changed to `bitmap`. The `bitmap` allocator allocates based on the first extent possible from the start of the disk, so large extents are preserved. It uses a fixed-size tree of bitmaps to track free space, thus using constant memory regardless of number of extents. As a result, the new allocator causes less fragmentation and requires less memory.