implement loadwallet and unloadwallet RPC commands

[hairetikos]

since zcash lacks this, we had to implement it

partially completes #101

for now, we support a single wallet being loaded, and unloaded

when loading a wallet again, a rescan is properly triggered -- this may be slow with some wallets, it is single-threaded rescan, we can consider multi-threaded upgrades in the future

however, if you load and unload and reload the same wallet, it will be fast once a single rescan has been done (ideal for when the quantum-resistant encryption is implemented)

functions that require a wallet (getbalance, etc), will fail gracefully

since this is complete, we can now work on full encryption using dm-crypt on linux, and veracrypt on mac/windows

Adds runtime wallet management via two new RPC commands. Supports single wallet at a time with secure memory wiping for Zcash/Zclassic shielded transaction types (Sprout and Sapling z-addresses).

loadwallet "filename"

• Verifies no wallet currently loaded and file exists
• Opens/verifies wallet database, creates CWallet instance
• Registers as validation interface
• Performs full blockchain rescan (required for z-addresses to rebuild witness caches)
• Re-enables mining if configured

unloadwallet

• Waits for pending async operations (z_sendmany, z_shieldcoinbase, z_mergetoaddress)
• Stops mining, flushes wallet to disk
• Securely wipes all cryptographic material via memory_cleanse():
  • Master encryption keys (vMasterKey)
  • HD seed, Sprout/Sapling spending keys, regular private keys
• Resets BerkeleyDB environment

Implementation

• CBasicKeyStore::CleanupKeys() - Clears all key maps (transparent, Sprout, Sapling)
• CCryptoKeyStore::CleanupKeys() - Wipes encrypted key material before clearing
• CWallet::CleanupForUnload() - Clears wallet state (transactions, nullifiers, metadata, master keys)

# Usage
zcl-cli unloadwallet
zcl-cli loadwallet "backup-wallet.dat"

Pattern follows existing pre_wallet_load() / post_wallet_load() in zcbenchmarks.cpp.

[hairetikos]

will re-open with just the relevant commits cherry-picked