CAS server: Implement BatchReadBlobs

This will allow more efficient artifact downloads on higher latency connections (see #554 (closed)) and is required for BuildBox prefetch support.

mentioned in commit f4c0092e

mentioned in merge request !785 (merged)

mentioned in commit 9ee57dcc

mentioned in commit f24061cb

mentioned in commit 1a7fb3cb

closed via merge request !785 (merged)

mentioned in commit b3ffcdc8

changed milestone to %BuildStream_v1.2

added Needs Backport Optimization labels

Re-opening for the backports to bst-1.2.

Note that #554 (closed) must be fixed in bst-1.2, please remember to backport the related patches.

reopened

mentioned in commit 3cd85f44

mentioned in commit 8f0bb875

Backport was merged in !814 (merged).

closed