Compute disk buffer initial_size with Jean's method

[blt]

@blt Actually, looking at the crate, there does appear to be a LevelDBIterator trait, which has a from (and to) method which seems to imply we can iterate over an arbitrary range of keys.

It seems to me we could create a DB handle, iterate X times, get the key of the last item, and set that as the starting key for the next DB handle iterator. Reading the offiial docs it appears key ordering happens lexicographically, which implies we can have stable ordering across DB handles at startup when the cache isn't actually in use, correct?

Originally posted by @JeanMertz in #7398 (comment)

[ktff]

This should solve the OOM issue reported in #6741 (comment).

cc. @mikhno-s

[blt]

We gave this method a shot in #7410 but the startup cost imposed by the technique is simply too high to be useful.