SingleFileStore can optimize space usage by coalescing adjacent free entry blocks.

This issue is related to issue ISPN-3877.
Here we have a use case where the size of the entries gradually keeps on increasing and then finally those entries expire.

1. Smaller entries expire and new bigger entries start getting added
2. These new bigger entries will not fit into the free slots (entries) created due to the expiry of smaller entries.
3. Thus, these new bigger entries only get allocated at the end of the file resulting in growth of file size.

The optimization proposed is to:
1. Coalesce/Combine adjacent free entries during the periodic purge cycle to create bigger/larger free entries.
2. While allocating from an existing free entry, check if the size of the free entry is more than required for the new request being allocated. If yes, then, split the free entry into two free entries:
a. First part equal to the length being requested
This is returned as a part of the allocation activity.
b. Second part equal to the remainder of the space.
This added back as a free entry to the freeList.

Thus free space created due to expiry of smaller entries becomes available for consumption for the larger entries to some extent.
This reduces file size of the store.