Compare index values at one method, across their representations

[noha]

A page read back from disk holds its values as byte arrays, a page written in this image and a journal entry hold SoilObjectIds, and the two are never equal. Four places compared them with #= and failed after a reopen:

• SoilIndexItemsPage>>#itemIndexForKey:value: and SoilSkipListIterator>>#findPageFor:value: - #removeKey:value: went into its absent block for an entry that is there, and adding a pair the index held appended a second entry for it, in a running database and in a replay.
• SoilAbstractBTreeDataPage>>#findItem:with: - the same miss, and the walk did not stop at the first page, so it read page 0 and failed on the stream position.
• SoilRemoveKeyEntry>>#isForItem:index: - a reader older than a removal lost the key, on unique indexes too.

All of them, and SoilAddKeyEntry>>#isForItem:index:, now ask SoilIndexItemsPage class>>#value:matches:. Across representations the bytes decide; within one, #= does, because an object id without an index yet (a new object before commit) only matches itself - as bytes all of them would look the same and a second new value under a key would overwrite the first.

Carries the red test of duplicate-key-remove-after-reopen (559587cb) and adds tests for the re-add, the replay, the MVCC restore of a removal on unique and duplicate keys, and the BTree walk past the first page. The replay test is skipped on BTree: SoilBTreePage>>#cleanRemovedIn: removes a tombstone by key and can take the live duplicate next to it, which is a fault of its own.