fix: relax mempool probes to 4 minutes

[openoms]

This is on signet:

May 17 15:09:10 [98] NOTICE: Starting Mempool Server... (57eddac)
May 17 15:09:11 [98] INFO: [Mining] Downloading pools-v2.json for the first time from https://raw.githubusercontent.com/mempool/mining-pools/master/pools-v2.json over clearnet
May 17 15:09:12 [98] INFO: Mining pools-v2.json import completed (no database)
May 17 15:09:12 [98] NOTICE: Mempool Server is running on port 8999
May 17 15:09:13 [98] NOTICE: The mempool is now in sync!
May 17 15:09:17 [98] INFO: Latest BTC fiat averaged price: {"time":0,"USD":26764,"EUR":24754,"GBP":21480,"CAD":36082,"CHF":24092,"AUD":40295,"JPY":3677626}

the

NOTICE: The mempool is now in sync!

is missing on mainnet and probably restarted before it can query everything from bitcoind

An other mainnet instance needs 28 seconds to sync on mainnet, but it has a cache.