More private sync using Electrum

[afilini]

The current syncing algorithm for Electrum (and Esplora) tries to minimize the number of requests made, with the disadvantage that the addresses are checked with the server sequentially, and the server could use that information to reduce the privacy of the user.

One mitigation could be to not ask for addresses sequentially and mix together both the external and internal addresses. This however makes it harder to find a sufficiently large gap without any utxo, which allows the sync to stop earlier, so it would probably translate to many more requests than what's necessary with the sequential process.

We could implement this with a flag that the user can switch to choose which "process" to use, the "fast" (but less private) one or the more private and slower one.