Log at debug level to file

EnvFilter is applied globally. This means you cannot log at INFO level
to the terminal and at DEBUG level to log files. To get a around this
limitation I had to implement the layer trait on a new type and filter
in the on_event() trait method. Each swap has its own log file denoted
by its swap_id. The logger appends to the existing file when resuming a
swap.

Closes #278

swap/src/cli.rs

@@ -1 +1,2 @@
 pub mod command;
+pub mod trace;