wallet.synchronizer should exist at construction.

The CLI command `electrum importprivkey` fails with the error message `Error: Keypair import failed: Wallet instance has no attribute 'synchronizer'`

The `self.synchronizer` attribute is accessed before it ever exists when using `electrum importprivkey` on the command line. This change sets it to None (which is a valid state for this attribute) in the wallet's constructor.