gridstore

New storage for variable-sized values using mmap.

Design

IDs are sequential integers, starting at 0.
The storage is divided into file pages of fixed size (32MB).
Data can be written and read across multiple pages.
Those pages are mapped into memory using mmap.
Data units are blocks of fixed size (128 bytes).
Values span an integer number of contiguous blocks.
Values are compressed with lz4
Each block is mapped to a bit in the bitmask.
A region is a fixed number of contiguous blocks.
Gaps of free blocks in each region are tracked in a file.
Deletes mark the block as deleted (in-memory) & updates their region
Updates:
- not done in place, always a new value is inserted
- calculation of the new regions gaps is done on the fly
- the tracker is updated in-memory, only persisted on flush
Supports multiple threads reading and single thread writing
One file per page, one file for tracker, one file for bitmask, and one file for gaps.