optimizations to BlobStorageEngine, ExpertLibrary

[fverac]

PR for multiple speedups to uploading and downloading with blob storage.

Changes:
BlobStorageEngine

1. Use async by default when creating a commit.
2. When uploading/ downloading multiple blobs, use only a single blob service client.
3. On batch upload/download, cache the last_modified value of a repo to avoid retrieving on every operation.
4. In async_create_commit, leverage async_upload_blobs instead of _async_upload_blob.

ExpertLibrary

1. Avoid redundant+expensive updating of readme.
2. optimize _build_lib for BlobStorageEngine