core/fetch: fetch height asynchronously
core/fetch: fetch height asynchronously

Fetch the generator height asynchronously so that the generator height
is updated even if the local Core is falling behind.

For chain/chainprv#1584.

Closes chain/chainprv#1597.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@boymanjor



chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain