LB healthchecks

[Yonik Seeley <ys...@gmail.com>]

I'm in the process of refactoring out a header.jsp for some of the
admin pages, and removing a little of the cruft.

Part of the legacy stuff is the hardcoded healthcheck file, the
existence of which signals that the server is "enabled" and that
traffic should be directed toward it from the loadbalancer.  I think
this is good stuff to keep, but it seems like the file location should
be configurable rather than being hardcoded in the JSPs (in multiple
locations no less).

So I'm planning on adding a healthcheck entry to the admin section of
solrconfig.xml like so:

<config>
  [...]
  <admin>
    [...]
    <healthcheck type="file">logs/server-enabled</healthcheck>
  </admin>
</config>

It leaves room for other healthcheck implementations by having a type attribute.

-Yonik