Add config option to enable httpCaching

HTTP Caching

Set HTTP caching related parameters (for proxy caches and clients).

The options below instruct Solr not to output any HTTP Caching
related headers

<httpCaching never304="true" />

If you include a directive, it will be used to
generate a Cache-Control header (as well as an Expires header
if the value contains "max-age=")

By default, no Cache-Control header is generated.

You can use the option even if you have set
never304="true"

       <httpCaching never304="true" >
         <cacheControl>max-age=30, public</cacheControl>
       </httpCaching>

To enable Solr to respond with automatically generated HTTP
Caching headers, and to response to Cache Validation requests
correctly, set the value of never304="false"

This will cause Solr to generate Last-Modified and ETag
headers based on the properties of the Index.

The following options can also be specified to affect the
values of these headers...

lastModFrom - the default value is "openTime" which means the
Last-Modified value (and validation against If-Modified-Since
requests) will all be relative to when the current Searcher
was opened. You can change it to lastModFrom="dirLastMod" if
you want the value to exactly correspond to when the physical
index was last modified.

etagSeed="..." is an option you can change to force the ETag
header (and validation against If-None-Match requests) to be
different even if the index has not changed (ie: when making
significant changes to your config file)

(lastModifiedFrom and etagSeed are both ignored if you use
the never304="true" option)

       <httpCaching lastModifiedFrom="openTime"
                    etagSeed="Solr">
         <cacheControl>max-age=30, public</cacheControl>
       </httpCaching>