blockstore: Reduce intermediate collects from multi_get()

[steviez]

Problem

The rocksdb multi_get() method allows us to fetch multiple values in a single rocksdb call. We currently return the results as a Vec by collecting item in LedgerColumn methods. But, we then immediately those Vec's, making the intermediate .collect() useless and wasteful.

Summary of Changes

• Move key allocation outside of multi_get() methods so the keys live long enough
• Make the multi_get() methods return impl Iterator ... and update call sites

[steviez]

Decent amount of whitespace change in this PR - I tried to keep cargo fmt runs in their own commits so reviewing commit-by-commit and/or hiding whitespace might reduce the noise

[bw-solana]

couple of questions. Looks like a nice improvement over all

[cpubot]

LGTM, just a note about potentially collecting rather than Vec::with_capacity()