Parallelize blob upload/download for restic copy

Currently restic copy will copy each blob from every snapshot serially,
which has performance implications on high-latency backends such as b2.

This commit introduces 8x parallelism for blob downloads/uploads which
can improve restic copy operations up to 8x for repositories with many
small blobs on b2.

This commit also addresses the TODO comment in the copyTree function.

Related work:

A more thorough improvement of the restic copy performance can be found
in PR #3513

changelog/unreleased/pull-3593

@@ -0,0 +1,9 @@
+Enhancement: Improve restic copy performance by parallelizing IO
+
+Restic copy previously only used a single thread for copying blobs between
+repositories, which resulted in limited performance when copying small blobs
+to/from a high latency backend (i.e. any remote backend, especially b2).
+Copying will now use 8 parallel threads to increase the throughput of the copy
+operation.
+
+https://github.com/restic/restic/pull/3593