Add minrelaytxfee settings

[comassky]

Hi,

Since few months, it's possible to push transaction with fees < 1 sat/vb.
For bitcoin core, it can be set with minrelaytxfee value (ex: -minrelaytxfee=0.0000010 means 0.1 sat/vb is min allowed).

There is a equivalent for Electrum / Floresta server ?

Thanks for helping :)