Survey key storage and usage of zcashd and zcash_client_sqlite

[str4d]

The ECC core team currently maintains two wallet codebases:

• The C++ full node wallet in zcashd, derived from Bitcoin Core's full node wallet.
• The Rust light client wallet in zcash_client_sqlite.

We want to unify these by building a backend-agnostic zcash_wallet in Rust, that can work with both zcashd and zcash_client_sqlite. However, these have quite different usages at present, and store their key material in different ways.

We should survey the zcashd and zcash_client_sqlite wallets, to document in a single location (here):

• How key material is generated.
• How generated key material is stored.
• How generated key material is exposed / can be exported.
• How key material can be imported.
• How imported key material is stored.
• How imported key material is exposed / can be exported.
• How key material is used within the wallet.