Cancel download and verify tasks when the mempool is deactivated

[mpguerra]

Motivation

If Zebra's network connection is interrupted for a long time, we'll want to deactivate the mempool while we're catching up to the new tip.

Designs

The download and verify service can call SyncStatus::is_close_to_tip to check if the mempool is inactive:

zebra/zebrad/src/components/sync/status.rs

Lines 42 to 43 in 44ac067

	/// Check if the synchronization is likely close to the chain tip.
	pub fn is_close_to_tip(&self) -> bool {

If the mempool is inactive, it needs to:

• cancel all downloads
• cancel all verifications
• return None to the caller

[mpguerra]

Hey team! Please add your planning poker estimate with ZenHub @conradoplg @dconnolly @jvff @oxarbitrage @teor2345 @upbqdn