[ADDED] Support for route S2 compression

[kozlovic]

The new field compression in the cluster{} block allows to
specify which compression mode to use between servers.

It can be simply specified as a boolean or a string for the
simple modes, or as an object for the "s2_auto" mode where
a list of RTT thresholds can be specified.

By default, if no compression field is specified, the server
will default to "accept", which means that a server will accept
compression from a remote and switch to that same compression
mode, but will otherwise not initiate compression. That is,
if 2 servers are configured with "accept", then compression
will actually be "off". If one of the server had say s2_fast
then they would both use this mode.

Here is the way to specify compression with a simple string:

cluster {
..
  # Possible values are "disabled", "off", "enabled", "on",
  # "accept", "s2_fast", "s2_better", "s2_best" or "s2_auto"
  compression: s2_fast
}

If the compression field is simply set to "s2_auto", then
the server will use default RTT thresholds of 10ms, 50ms
and 100ms for the "uncompressed", "fast", "better"
and "best" modes.

To specify a different list of thresholds for the s2_auto,
here is how it would look like:

cluster {
..
  compression: {
    mode: s2_auto
    # This means that for RTT up to 5ms (included), then
    # the compression level will be "uncompressed", then
    # from 5ms+ to 15ms, the mode will switch to "s2_fast",
    # then from 15ms+ to 50ms, the level will switch to
    # "s2_better", and anything above 50ms will result
    # in the "s2_best" compression mode.
    rtt_thresholds: [5ms, 15ms, 50ms]
  }
}

If a server has compression mode set (other than "off") but
connects to an older server, there will be no compression between
those 2 routes.

Signed-off-by: Ivan Kozlovic ivan@synadia.com

[derekcollison]

LGTM

[derekcollison]

LGTM