fix: do not start LDK unless we have fresh fee and block data

[limpbrains]

Description

• as discussed with @Jasonvdb we are now always trying to get fresh fees for LDK. Never use cached value. We try mempool and blocktank. Both sources two times before we fail. Also with 10 seconds timeout. If function fails, it throws an error, LDK needs to process it.
• same for blockdata. We check that electrum is connected and block data is not the default one.
• we do not start LDK unless we make sure both getFees and getBestBlock works
• getFees is covered with unit tests
• simplified onchain fee fetching logic for Bitkit, as it is covered by beignet
• show error if LDK start / refresh fails
• react-native-ldk probably needs to be updated to handle possible exceptions @Jasonvdb

Linked Issues/Tasks

closes #2352

Type of change

Refactoring

Tests

Unit test

QA Notes

Everything should work as before. Just LDK should not start if network is down and should show an error