feat: add an optional predicate to table iteration

[huitseeker]

As this is liable to overload memory, the present PR introduces an optional Predicate argument (a Fn(&(Key, Value)) -> bool) which allows filtering a subset of the table's items instead of the whole table.

[mystenmark]

Seems fine to me, although I wonder why we don't just have Iter return a stream, so that the caller can turn it into an iterator and use the full panoply of iterator methods on the result. ("I don't have time for that right now" is a fine answer.)

[huitseeker]

although I wonder why we don't just have Iter return a stream, so that the caller can turn it into an iterator and use the full panoply of iterator methods on the result.

"Use the panoply of iterator methods" is confusing me: are you calling for a return of an fully-in-memory object that happens to have iterator semantics (in which case I would note HashMap implements IntoIterator)? Or are you instead calling for an Iterator object that would happen to stream values in memory, one by one?

If the later, the answer of why this is difficult (aka "I don't have time") is because:

1. Iterator is a trait object, so it's probably much easier to return this as a method on rocksdb::DBMap than trait::Map,
2. we want this to be accessible from an async method (.iter), which by pattern requires the return going through a channel, which requires pretty annoying constraints on the pointer to the iterator (essentially pinned stack allocation),
3. and then the Iterator itself does not yield asynchronously.

Past those trifling details I suspect your mind is in the right place, though and that we want to turn this API into one that returns a futures::Stream. Want to open an issue for that?

/cc @laura-makdah for why this API may end up deprecated soon-ish, and what it may be replaced by.

[mystenmark]

Its not important to do this right now - what i should have suggested is using a tokio mpsc channel, which can be adapted by the caller into a stream, which works fine async.

[lanvidr]

This is a good intermediate solution, thanks!