interfaces: move peer `capabilities()` to the `ipeercapabilities` interface...
interfaces: move peer `capabilities()` to the `ipeercapabilities` interface
I'm not sure why this was on the `ipeercommands` interface. It appears to be
because these interfaces started out as `_basewirecommands` to hold wire
commands, back in 558f5b2ee10e. The capabilities interface wasn't split out
until 98861a2298b5, when it pulled the capability related methods off of the
`ipeerbase` interface. Perhaps it was an oversight to not look at the commands
interface because, while this is a wire command, both `sshpeer` and `httppeer`
now perform a handshake while instantiating the peer object, and cache a fixed
list of capabilities in that object. Likewise, `localpeer` is given a fixed set
of capabilities when instantiated. Back in 558f5b2ee10e, `httppeer` looks like
it issued a wire command when this method was called, but `sshpeer` obtained and
cached the capabilities when instantiated, and this method always returned a
fixed value.
There's a perfectly good interface with other capability related methods, and
having it here makes it easier to implement the base `peer` mixin class.
