commands: add changegaplimit and getminacceptablegap cmds

There are some legitimate use-cases for changing the gap limit,
so just as it has already been possible to do in the GUI (using the console only! don't expose this to mere mortals.),
now CLI/RPC users have this exposed too.

The "changegaplimit" command will always raise unless invoked with the --iknowwhatimdoing option.

closes #5882

e.g.:
$ curl --data-binary '{"id":"curltext","jsonrpc":"2.0","method":"changegaplimit","params":{"new_limit": 30, "iknowwhatimdoing": true, "wallet":"~/.electrum/testnet/wallets/test_segwit_2"}}' http://user:password@127.0.0.1:7777
$ ./run_electrum --testnet changegaplimit 30 --iknowwhatimdoing -w ~/.electrum/testnet/wallets/test_segwit_2

electrum/commands.py

@@ -49,7 +49,7 @@ from .transaction import (Transaction, multisig_script, TxOutput, PartialTransac
                           tx_from_any, PartialTxInput, TxOutpoint)
 from .paymentrequest import PR_PAID, PR_UNPAID, PR_UNKNOWN, PR_EXPIRED
 from .synchronizer import Notifier
-from .wallet import Abstract_Wallet, create_new_wallet, restore_wallet_from_text
+from .wallet import Abstract_Wallet, create_new_wallet, restore_wallet_from_text, Deterministic_Wallet
 from .address_synchronizer import TX_HEIGHT_LOCAL
 from .mnemonic import Mnemonic
 from .lnutil import SENT, RECEIVED
@@ -794,6 +794,30 @@ class Commands:
         """Create a new receiving address, beyond the gap limit of the wallet"""
         return wallet.create_new_address(False)
 
+    @command('w')
+    async def changegaplimit(self, new_limit, iknowwhatimdoing=False, wallet: Abstract_Wallet = None):
+        """Change the gap limit of the wallet."""
+        if not iknowwhatimdoing:
+            raise Exception("WARNING: Are you SURE you want to change the gap limit?\n"
+                            "It makes recovering your wallet from seed difficult!\n"
+                            "Please do your research and make sure you understand the implications.\n"
+                            "Typically only merchants and power users might want to do this.\n"
+                            "To proceed, try again, with the --iknowwhatimdoing option.")
+        if not isinstance(wallet, Deterministic_Wallet):
+            raise Exception("This wallet is not deterministic.")
+        return wallet.change_gap_limit(new_limit)
+
+    @command('wn')
+    async def getminacceptablegap(self, wallet: Abstract_Wallet = None):
+        """Returns the minimum value for gap limit that would be sufficient to discover all
+        known addresses in the wallet.
+        """
+        if not isinstance(wallet, Deterministic_Wallet):
+            raise Exception("This wallet is not deterministic.")
+        if not wallet.is_up_to_date():
+            raise Exception("Wallet not fully synchronized.")
+        return wallet.min_acceptable_gap()
+
     @command('w')
     async def getunusedaddress(self, wallet: Abstract_Wallet = None):
         """Returns the first unused address of the wallet, or None if all addresses are used.
@@ -1098,6 +1122,7 @@ command_options = {
     'fee_level':   (None, "Float between 0.0 and 1.0, representing fee slider position"),
     'from_height': (None, "Only show transactions that confirmed after given block height"),
     'to_height':   (None, "Only show transactions that confirmed before given block height"),
+    'iknowwhatimdoing': (None, "Acknowledge that I understand the full implications of what I am about to do"),
 }