frost-server and frost-client audit

[mpguerra]

This issue tracks completion of the audit on the frost-server and frost-client elements of the FROST for zcash demo.

[conradoplg]

Proposed scope (paths may change after #373 )

• server/ - everything
• frost-client/ - everything, but less priority (we might be able to restrict the scope here if by the time the audit starts we have more clearly split the "library" part from the "binary" part of the crate per Refactor/organize the whole project, e.g. move participant and coordinator to inside frost-client. Possibly remove their old CLI interface? #380 )
• participant/ - only the parts imported by frost-client/ (i.e. cli_for_processed_args() and all functions it calls when using HTTPComms)
• coordinator/ - only the parts imported by frost-client/ (i.e. cli_for_processed_args() and all functions it calls when using HTTPComms)

Parts that we believe should receive more focus:

• Server authorization and authentication, overall security
• Client end-to-end-encryption