Bug 28276: Do not fetch config ($KOHA_CONF) from memcached

Bug 28276: Do not fetch config ($KOHA_CONF) from memcached

memcached address and namespace are in $KOHA_CONF, so it is required to
read it before being able to access the cache. And after that,
configuration is kept in memory forever. Storing this in memcached is
useless and even counter-productive, since Koha reads both the file and
the cache

This patch addresses this issue by removing the cache-related code from
C4::Context->new.
It means that C4::Context->new will always read the configuration file,
so this patch also replaces inappropriate calls to
C4::Context->new->config by appropriate calls to C4::Context->config

It also fixes a bug where C4::Context->new would ignore the filepath
given in parameters if there was something in cache.

It also removes a problematic call to Koha::Caches->get_instance.
Because this call was outside of any subroutine, it would have happened
before the initialization of $C4::Context::context (which happen in
C4::Context::import)

Test plan:
1. Do not apply the patch yet
2. Add the following line at the beginning of Koha::Config::read_from_file
    warn "read_from_file($file)";
   This will allow you to check how many times the file is read.
3. Flush memcached and restart starman
4. Check the logs, you should see "read_from_file" a bunch of times
5. Apply the patch
6. Re-add the line from step 2
7. Flush memcached and restart starman
8. Check the logs, you should see "read_from_file" only once
9. Make sure the memcached config from $KOHA_CONF (memcached_servers,
   memcached_namespace) is taken into account by checking the About page

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>