use `CidHashMap` in the plain CAR-file blockstore

[lemmih]

Issue summary

The blockstore backed by an uncompressed CAR file uses two HashMaps with CIDs as keys: One map for the write cache and one map for the index. These maps should be replaced with CidHashMap, which uses less memory. Memory usage when validating a snapshot should be measured before and after the change.

Acceptance criteria:

• Add missing methods to CidHashMap
• Replace HashMap<Cid, V> with CidHashMap<V> in car/plain.rs
• Measure change in peak memory use when validating an uncompressed calibnet and mainnet snapshot. See /usr/bin/time -v forest-cli snapshot validate {snapshot}.car

Other information and links

forest/src/db/car/plain.rs

Lines 152 to 153 in 6dbd776

	write_cache: ahash::HashMap<Cid, Vec<u8>>,
	index: ahash::HashMap<Cid, UncompressedBlockDataLocation>,