Bug 9434: Introduce new tied cache system

At the moment we cache numerous pieces of information in module-level
variables which then do not get updated in other threads/processes when
they are changed by the user. This is a serious usability issue.

Examples of this include the way we treat sysprefs (there is now a
method to disable the syspref cache, but by default it is enabled),
notices, frameworks, field mappings, and koha-conf.xml, at least.

This patch sets the stage for eliminating this problem by making it
possible to convert module-level cache variables into variables that
are actually backed by whatever caching system may be configured. This
is done through a special Koha::Cache::Object class which can be tied
to the variables that are being used for caching and provided with a
constructor method/closure to allow the cache to be reloaded when it
expires. For example:

    my $cache    = Koha::Cache->new();
    my $data     = 'whatever';
    my $variable = Koha::Cache->create_scalar(
        {
            'key'         => 'whatever',
            'timeout'     => 2,
            'constructor' => sub { return $data; },
        }
    );
    print "$$variable\n"; # Prints "whatever"

The one change this necessitates for accessing the data is that the
variable must be dereferenced an additional time before use (i.e.
$$variable instead of $variable). There is no difference when the
variable tied is a hash (created with Koha::Cache->create_hash). This
is a small price to pay for Koha working in a multi-threaded, persistent
environment. This change will also make caching easier in general.

CHI was incompatible with the variable tying, so this patch also removes
the dependency on CHI, using instead Cache::Memcached::Fast,
Cache::FastMmap, and Cache::Memory, when they are available.

To test:
1) Apply patch.
2) Run unit test t/Cache.t (after setting the MEMCACHED_SERVERS and
   CACHING_SYSTEM environment variables). As no changes were made to
   the tests already in that file, this passing demonstrates there
   are no regressions.
3) With memcached caching enabled (you must set the MEMCACHED_SERVERS
   and CACHING_SYSTEM environment variables) and DEBUG turned on (i.e.
   the DEBUG environment variable set to 1), try running a report via
   the web service ([intranet]/cgi-bin/koha/svc/report?id=1 and check
   your web server logs to confirm that there are messages like
   "get_from_cache for intranet:report:id:1" in them.
4) If the reports worked, sign off.

NOTE: Technically you could test this without needing memcached by
installing libcache-fastmmap-perl and setting CACHING_SYSTEM to
'fastmmap' instead of 'memcached'. You could also install libcache-perl
and set CACHING_SYSTEM to 'memory' but there would be little point as
the cached variables would go out of scope in between runs.
Bug 8089: Cache sysprefs with Koha::Cache::Object

This patch replaces the %sysprefs hash variable with a
Koha::Cache::Object-tied hash that allows the user to cache sysprefs
in memcached, fastmmap, or memory.

To test:
1) Enable the cache as described on bug 9434.
2) Try viewing some pages in the OPAC and staff client. If the syspref
   caching isn't working, you'll be redirected to the web installer
   (in the staff client) or be shown the maintenance page (in the OPAC).
3) Disable the cache by unsetting the CACHING_SYSTEM environment
   variable.
4) Try viewing some pages in the OPAC and staff client. If the syspref
   caching isn't working, you'll be redirected to the web installer
   (in the staff client) or be shown the maintenance page (in the OPAC).

Note that this will not improve performance. It does, however, eliminate
the problem of sysprefs not getting updated in all threads in
multi-threaded environments.