Memory allocation problems with multiple wallets

Steps to reproduce:
- create 10 multisig wallets (i.e. 3 of 6)
- enjoy the problems with memory allocation

Expected behaviour:
- should work with arbitrary number of wallets

Solution:

Don't keep all wallets in RAM (only keep names), load them as they are needed. This will slow down PSBT parsing, but will work with any number of wallets. To speed up things maybe makes sense to off-load them to SDRAM instead of MCU internal RAM.

Start by reproducing the problem with 10 multisig wallets and observe the memory allocation failure. Trace how wallets are retained in RAM and how PSBT parsing loads them, then verify that wallets can be created and used in arbitrary numbers without exhausting MCU memory.

Written by the indexing model from the issue text.