Bug 36736: Load plugins at the start of background job processing

Bug 36736: Load plugins at the start of background job processing

This patch adds a call to get_enabled_plugins before processing background jobs to ensure
that all plugin hooks are loaded and cached

To test:
1 - Install a plugin that adds new objects e.g. the Contracts plugin
    https://github.com/bywatersolutions/fs-koha-plugin-contracts
    or the Koha Advent plugin:
    https://gitlab.com/koha-community/koha-advent/koha-plugin-fancyplugin
2 - Restart all
3 - Tail all your logs
4 - Stage and import a file containing items
5 - Note in the logs
    DBI Exception: DBD::mysql::st execute failed: Lock wait timeout exceeded; try restarting transaction
6 - Apply this patch
7 - Restart all
8 - Stage and import again
9 - Success!

Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>