Allow using nodes in pruned mode

[cmihai]

When starting a node in pruned mode (-prune=1 -isd=1, for example), then trying to create a new wallet by importing a mnemonic, the wallet creation fails. The reason is that, by default, the master key import triggers a rescan of the blockchain, which is disabled in pruned mode.

A solution would be to retry the import with rescan disabled, if the RPC call fails the first time.