Rollup node: protect GC and block processing with locks

Context

This MR locks the GC and the block processing of the rollup node behind 2 different locks. These locks will always be released before they need to be taken again with the current implementation but this change is necessary for preparing the snapshotting functionality. These locks will be used in !10704 (merged) to prevent the rollup node from writing when a snapshot is being exported.

Manually testing the MR

CI.