Consider evmap for use in storage

[Thomasdezeeuw]

Currently we use a simple HashMap for storing the uncommitted/stored/removed blobs. However this requires a RPC with the storage actor on each request. Using evmap this could be removed.

Related code:

stored/src/storage/mod.rs

Lines 255 to 266 in c7b3416

	/// All blobs currently in [`Index`] and [`Data`].
	///
	/// # Safety
	///
	/// `blobs` must be declared before `data` because it must be dropped before
	/// `data`.
	// TODO: use different hashing algorithm.
	blobs: HashMap<Key, (EntryIndex, BlobEntry), FxBuildHasher>,
	/// Uncommitted blobs.
	///
	/// The `usize` is the number of [`StoreBlob`]s adding the blob.
	uncommitted: HashMap<Key, (usize, UncommittedBlob), FxBuildHasher>,