commands: add changegaplimit and getminacceptablegap cmds
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

wakiyamap/electrum-mona,spesmilo/electrum,spesmilo/electrum,spesmilo/electrum,pooler/electrum-ltc,spesmilo/electrum,vialectrum/vialectrum,pooler/electrum-ltc,wakiyamap/electrum-mona,vialectrum/vialectrum,wakiyamap/electrum-mona,vialectrum/vialectrum,wakiyamap/electrum-mona,pooler/electrum-ltc,pooler/electrum-ltc