Add Network observer attack: Identify Internet user as a Bitcoin user

[kristovatlas]

Thinking about what may be leaked by using a full node client without a private network to obfuscate IP address, this is one of the things betrayed.

Countermeasure: Tor/VPN/etc.

[dcousens]

Was relating "bitcoin activity time" to "corresponding transactions in the mempool at that time" an attack?
I know we had similar, but I think it was related to matching it against an existing set of related addresses, not "all transactions in the mempool".