Follow best practices for documenting the exported pieces of the
Snowflake client library.

Add exported functions to the snowflake client library to allow calling
programs to define and set their own custom broker rendezvous methods.

Now the geoip implmentation has being moved to it's own library to be
shared between projects.

The broker is a global object.

Parsing the Snowflake client options from SOCKS allow us to specify
snowflake client settings in the bridge lines.

This should increase the maximum amount of inflight data and hopefully
the performance of Snowflake, especially for clients geographically
distant from proxies and the server.

Currently the same as httpRendezvous, but activated using the -ampcache
command-line option.

This package contains a CacheURL function that modifies a URL to be
accessed through an AMP cache, and the "AMP armor" data encoding scheme
for encoding data into the AMP subset of HTML.

Makes BrokerChannel abstract over a rendezvousMethod. BrokerChannel
itself is responsible for keepLocalAddresses and the NAT type state, as
well as encoding and decoding client poll messages. rendezvousMethod is
only responsible for delivery of encoded messages.

Formerly, BrokerChannel represented the broker URL and possible domain
fronting as
	bc.url  *url.URL
        bc.Host string
That is, bc.url is the URL of the server which we contact directly, and
bc.Host is the Host header to use in the request. With no domain
fronting, bc.url points directly at the broker itself, and bc.Host is
blank. With domain fronting, we do the following reshuffling:
	if front != "" {
		bc.Host = bc.url.Host
		bc.url.Host = front
	}
That is, we alter bc.url to reflect that the server to which we send
requests directly is the CDN, not the broker, and store the broker's own
URL in the HTTP Host header.

The above representation was always confusing to me, because in my
mental model, we are always conceptually communicating with the broker;
but we may optionally be using a CDN proxy in the middle. The new
representation is
	bc.url   *url.URL
        bc.front string
bc.url is the URL of the broker itself, and never changes. bc.front is
the optional CDN front domain, and likewise never changes after
initialization. When domain fronting is in use, we do the swap in the
http.Request struct, not in BrokerChannel itself:
	if bc.front != "" {
		request.Host = request.URL.Host
		request.URL.Host = bc.front
	}

Compare to the representation in meek-client:

https://gitweb.torproject.org/pluggable-transports/meek.git/tree/meek-client/meek-client.go?h=v0.35.0#n94
	var options struct {
		URL       string
		Front     string
	}
https://gitweb.torproject.org/pluggable-transports/meek.git/tree/meek-client/meek-client.go?h=v0.35.0#n308
	if ok { // if front is set
		info.Host = info.URL.Host
		info.URL.Host = front
	}

The tests were using a broker URL of "test.broker" (i.e., a schema-less,
host-less, relative path), and running assertions on the value of
b.url.Path. This is strange, especially in tests regarding domain
fronting, where we care about b.url.Host, not b.url.Path. This commit
changes the broker URL to "http://test.broker" and changes tests to
check b.url.Host. I also added an additional assertion for an empty
b.Host in the non-domain-fronted case.

Closes: #40055

Simplify the code to use a ticker. Using a pattern to allow a first run
of the loop before hitting the ticker:
https://github.com/golang/go/issues/17601#issuecomment-311955879

Instead of copying the value.

Unused since 1364d7d4.

The legacy code does case matching on these exact strings so it's better
to ensure they're constant.