Enable wallet key imports without rescan in pruned mode.

Complete rescan is incompatible with pruning, but rescan is optional on
our wallet key import RPCs. Import on use is very useful in some common
situations in conjunction with pruning, e.g. merchant payment tracking.

This reenables importprivkey/importaddress/importpubkey when rescan
is not used.

In the future we should consider changing the rescan argument to allow depth
or date to allow limited rescanning when compatible with the retained
block depth.