Fsa/new version management

[finnschiermer]

Reclaim intermediate (frozen) versions.

This PR holds two major upgrades

• The lock-file gets a new updated format which allows for separate tracking of frozen and live versions. It also drastically simplifies the machinery (now called the VersionList) by using plain locking instead of a fancy lock-free data structure.
• The "backdating" algorithm is now part of every commit. It looks for file space which is not reachable through any live or frozen version and recycles it for new allocations, thereby eliminating most of the file growth otherwise caused by version pinning.

Testing is done by two random scenarios, one at Core Storage level, one at ObjectStore level. The tests generate scenarios with random use of both live version pinning, frozen version pinning and dropping of versions. The test at ObjectStore level uses notifications. In debug mode every use of the backdating algorithm includes verification that a) recycled blocks are not referenced and b) kept blocks are indeed referenced. This validation is expensive, so only enabled in debug mode.

The complexity is O(free-list-size * log2(max(versions,free-list-size)) so it may add CPU overhead to commit. However, recycling blocks earlier often leads to less fragmentation and shorter free-lists. The observed net result for the tests is a clear performance win.

[RedBeard0531]

I think this will be the last roundtrip

[RedBeard0531]

🚢🇮🇹!

(after fixing a minor typo)

[ironage]

In case you missed it, there are 20 new warnings reported on CI "Code will never be executed" around calls to ALLOC_DBG_COUT in debug mode when REALM_ALLOC_DEBUG is defined as 0.