Fix importprivkey / rescan

importprivkey / rescan does not always work currently.

Use case:
- dumpprivkey with a balance
- close bitcoin
- delete wallet.dat
- start bitcoin
- importprivkey

Result:
rescan fails because wallet uses nTimeFirstKey of the other keys.
Setting
    pwalletMain->nTimeFirstKey = 1;
solves the problem in this case, scanning the chain from genesis.
But I if import with rescan=false, then close bitcoin and do
    bitcoin-qt -rescan
this fails again. Because nCreateTime of the metadata is zero,
and now nTimeFirstKey is set to zero, but overwritten later again,
because if(0) is false. So setting this to 1 solves the problem.

Further ideas, not part of this pull:
dumprivkey could export nCreationTime as well.
importprivkey would then have distinguish between old and new format.
rescan from the genesis block could reassign all nCreationTime.