irkerd: Pull request-parsing out into Irker._parse_request

irkerd: Pull request-parsing out into Irker._parse_request

There is a lot of error checking here, which is good, but it distracts
from the core logic of Irker.handle.  By pulling the parsing out into
a private helper function, we isolate the code focused on parsing and
error checking from the code focused on dispatching and connection
management, making both easier to read.

I've also changed the Target-validation logic.  The old Target.valid
returned True if the Target URL was valid, and False otherwise.  The
new Target.validate returns None, and raises an InvalidRequest
exception with an error message describing exactly why the URL is
invalid.  We print these messages when dropping server URLs in
Irker._parse_request, while the old Irker.handle code silently dropped
invalid targets.  We also continue processing other server URLs after
an invalid Target, while the old Irker.handle code bailed out after
the first invalid Target.  Besides making the invalid URLs more
obvious in the logs and increasing resiliency to invalid URLs, these
changes allow us to pull the URL-to-Target conversion out of
Irker.handle entirely, so it can focus more strongly on dispatch and
connection management.