[typed_store] Write an iterator-based variant of multi_get

[huitseeker]

• multi_get was added in Added multi_get and Iter seek #2
• its argument is a slice of values, leading to value copies, because we serialize each of these before passing them to the DB:
  multi_get(&self, keys: &[K]) -> Result<Vec<Option<V>>>
• We should instead take an iterator with references or values for keys at Item (std::borrow::Borrow is useful here),
• inspiration can be gleaned by the type of the delete_batch function, which takes the same sort of argument:
  

  mysten-infra/typed-store/src/rocks/mod.rs

  Lines 164 to 168 in 5221b8f

  	pub fn delete_batch<J: Borrow<K>, K: Serialize, V>(
  	mut self,
  	db: &DBMap<K, V>,
  	purged_vals: impl IntoIterator<Item = J>,
  	) -> Result<Self, TypedStoreError> {

• this change should then be propagated downstream to packages using this.

[huitseeker]

This was done! https://github.com/MystenLabs/mysten-infra/pull/38/files#diff-b360df397831a1d4239f8dfc8335392f602f32e2d192f0507f60bb55f73b63f1

[akichidis]

This was done! https://github.com/MystenLabs/mysten-infra/pull/38/files#diff-b360df397831a1d4239f8dfc8335392f602f32e2d192f0507f60bb55f73b63f1

oh nice, two birds one stone! :)