Reporter: potential out-of-mem issue when btclightclient is far behind BTC (not-urgent)

[gitferry]

The bootstrapping mechanism of the reporter is subject to potential out-of-mem issue because it caches all the BTC blocks from the base height to the latest. It will be an issue if the btclightclient module of Babylon is far behind BTC a lot. This situation is not urgent for now because the liveness of Babylon requires at least a reporter running 24/7. Still, it will be critical if the reporter is down for a long time because we might not be able to restart the reporter due to the out-of-mem issue.

cc @SebastianElvis @gusin13.