Investigate possible optimisations for managed wallets fees

[ygrishajev]

Fees related to managed wallets transactions are taking up quite some funds. We'd like to investigate possible options to optimise those.
The main file where transactions are happening is BatchSigningClientService.

One possible optimisation might be putting all the batched messages into a single transaction. The downside of it is that if one tx fails other will too. So some ways to mitigate/retry these might be needed.