• Add AddWatchOnlyWithDB, AddKeyOriginWithDB, AddCScriptWithDB functions

AddWatchOnlyWithDB, AddKeyOriginWithDB, and AddCScriptWithDB add their
respective data to the wallet using the provided WalletBatch instead
of creating a new WalletBatch object every time. This allows for batching
writes to the database.

• Have WalletBatch automatically flush every 1000 updates

Since it now automatically flushes, we don't need to have
UpgradeKeyMetadata count and flush separately

• Batch writes for importmulti

When writing all of the imported data to the wallet, use a common
WalletBatch object so that batch writes are done and the writes
finish more quickly.

AddKeypoolPubkey is no longer needed so it is also removed

• Move some of ProcessImport into CWallet::Import*

This maintains encapsulation of CWallet::database in the face of
batching, e.g. allows making the WithDB methods private.

• Apply the batch treatment to CWallet::SetAddressBook via ImportScriptPubKeys
• wallet: Pass WalletBatch to CWallet::UnsetWalletFlag

This is a backport of Core PR15741

I did it all in one block even though it's quite large because intermediate commits do not work individualy.

Depends on D6385