[sovereign] Bugfix notifier bootstrapper + cleanup notarized headers

[mariusmihaic]

Reasoning behind the pull request

• Several bugs

Proposed changes

• Fixed notifier bootstrapper to use a confidence threshold for synced times before subscribing to receive data. This is because the sync mechanism can sometimes send "false positives"
• Fixed cleanup of notarized headers for cross shard. There was a very tricky bug, which would delete notarized mainchain headers instead of self chain headers

Testing procedure

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?

[axenteoctavian]

Suggested change

	} else if round == 2 { // first cross notarized header is 10000000
	require.Equal(t, headerNonce, crossNotarizedHeader.GetNonce())