Max-buy and spread is not something that one would configure on every run.
More convenient to keep this in the config.
The max-buy Bitcoin value was adapted to `0.02` which is more reasonable for mainnet.
Activated feature `serde-float` to serialize the spread (Decimal) as float instead of string.
```
...
[maker]
max_buy_btc = 0.02
ask_spread = 0.02
```
#[structopt(about = "Main command to run the ASB.")]
Start{
#[structopt(long = "max-buy-btc", help = "The maximum amount of BTC the ASB is willing to buy.", default_value = "0.005", parse(try_from_str = parse_btc))]
max_buy: Amount,
#[structopt(
long="ask-spread",
help="The spread in percent that should be applied to the asking price.",
default_value="0.02"
)]
ask_spread: Decimal,
#[structopt(
long="resume-only",
help="For maintenance only. When set, no new swap requests will be accepted, but existing unfinished swaps will be resumed."