New client-side filters

[lontivero]

At the time of writing this, the Wasabi client-side filters are 250MB size, almost 200 megabytes bigger than a year ago. At this grow rate sooner or later it will be necessary to do something to reduce its size.

The easiest/cheaper way to cut the size at a half is by saving them in binary instead of using the hex encoding. This was done and tested by me and it is a proven solution that can reduce the current 250MB to 125MBs with almost no pain.

Even doing that, it is expected that in about year we will have 250MB size again. What we can do then is batching the filters as described here:
https://github.com/bitaps-com/bips/blob/master/bip-block-batch-filters.mediawiki this is not a problem for us because we don't need to be compatible with Bitcoin Core filters.

[nopara73]

Applying Private Information Retrieval to Lightweight Bitcoin Clients
PRESENTER(s):
Kaihua Qin, Henryk Hadass, Arthur Gervais (Imperial College London), Joel Reardon (University of Calgary)

[MaxHillebrand]

FYI, here a good writeup by @kallewoof
https://bc-2.jp/bfd-profile.pdf