The mempool load can take a while, and it is not
needed for the indexes' synchronization.
Also, having the mempool load function call
inside 'blockstorage.cpp' wasn't structurally
correct.
This is a partial backport of core#27607
https://github.com/bitcoin/bitcoin/pull/27607/commits/ed4462cc78afd2065bbf5bd79728852b65b9ad7f