Tiny balance update bug after importprivkey

[hebasto]

If you import a private key of an address you are already watching and without a rescan, the available balance does not upgrade correctly.

Steps (regtest):

1. generate 101
2. getnewaddress, store somewhere
3. sendtoaddress <addr-from-2> 10
4. dumpprivkey <addr-from-2>, store somewhere
5. Stop node
6. Remove wallet.dat
7. Start node
8. importaddress <addr-from-2> (== triggers rescan) watch only balance should list properly
9. importprivkey "<WIF-key-from-4>" "test" false will lead to available balance of 0 (but should be 10)

A restart solves the issue.

Bitcoind (listtransactions, getbalance, etc.) is not affected.

Moved from bitcoin/bitcoin#11477.