Implement better private key import RPC extension. about btcwallet

[arnuschky]

As a workaround for issue 2), I tried the following:
When batch-importing, I used importprivkey <key> <label> false to avoid triggering multiple rescans.

My idea was that I could trigger a rescan afterwards manually. But alas, there is apparently no way to trigger a rescan (see #250)

By the way, after importing the keys as indicated above, btcwallet starts up like as follows:

14:00:35 2015-04-18 [INF] WLLT: Started rescan from block 00000000f952d926901f537502c834baa9790a8a178adc46ec9d7c82cb5d5bf0 (height 349578) for 14 addresses
14:00:35 2015-04-18 [INF] WLLT: Finished rescan for 14 addresses (synced to block 00000000f952d926901f537502c834baa9790a8a178adc46ec9d7c82cb5d5bf0, height 349578)

[arnuschky]

For completeness, here the hint from @jrick how to trigger a rescan manuall:

On master (which uses the old txstore package) this can be triggered by closing wallet, deleting the file ~/.btcwallet/{main,test,sim}net/tx.bin, and restarting

[jrick]

Rather than adding an extension to the legacy RPC server, these improvements should be added to the walletrpc.WalletService/ImportPrivateKey RPC.