wallet: Change default confirm target from 2 to 6

Recent discussion (in IRC meetings, and e.g. bitcoin#8989) has shown a
preference for the default confirm target for smartfees to be 6 instead
of 2, to avoid overpaying fees for questionable gain.

6 is also a compromise between the GUI's pre-bitcoin#8989 value of 25 and the
bitcoind `-txconfirmtarget` default of 2. These were unified in bitcoin#8989,
but this has made the (overly expensive) default of 2 as GUI default.

Author: laanwj

src/wallet/wallet.h

@@ -53,7 +53,7 @@ static const bool DEFAULT_SPEND_ZEROCONF_CHANGE = true;
 //! Default for -sendfreetransactions
 static const bool DEFAULT_SEND_FREE_TRANSACTIONS = false;
 //! -txconfirmtarget default
-static const unsigned int DEFAULT_TX_CONFIRM_TARGET = 2;
+static const unsigned int DEFAULT_TX_CONFIRM_TARGET = 6;
 //! -walletrbf default
 static const bool DEFAULT_WALLET_RBF = false;
 //! Largest (in bytes) free transaction we're willing to create