Allow setting a custom backend storage

[esolitos]

With this change the secondary storage bin can be customised on demand, allowing to define, for example, another memory storage.
This is can be particularly useful in conjunction with wodby/drupal-varnish#3 because it allows the bin to be completely customised.

Note This is an extension of VARNISHD_STORAGE_SIZE, making it more customisable while preserving backward compatibility.

[csandanov]

I think it would be better to use something like VARNISHD_SECONDARY_STORAGE where you can specify storage name, type, path and size.

[esolitos]

I agree, that VARNISHD_SECONDARY_STORAGE is a better name for the variable (changed it now), however I was thinking to leave the storage name fixed to secondary, just to prevent the need for a second variable for the storage name in the use-case: wodby/drupal-varnish#3
However if you think it's better leaving the storage name configurable as well, I'll change that as well.

[csandanov]

What I meant is to have just a single env var VARNISHD_SECONDARY_STORAGE instead of two env vars. As the example value it could be:

VARNISHD_SECONDARY_STORAGE=file,/var/lib/varnish/storage.bin,10MiB

Because now variable VARNISHD_STORAGE_SIZE actually means "add additionall storage with type file, named secondary, with the path /var/lib/varnish/storage.bin and with the size what have specified". We can keep name secondary because it will be used in the variable name.

[esolitos]

Ok, the idea was to leave VARNISHD_STORAGE_SIZE for backward compatibility, do you think I should simply drop it?

Last commits are just small changes: renamed the variable and changed the priority since the VARNISHD_STORAGE_SIZE would be deprecated.

[csandanov]

Ohh, I'm sorry, I didn't realize we had it before :) yes, I think we should drop it, it's stupid

[esolitos]

Updated dropping VARNISHD_STORAGE_SIZE in favour of the more generic VARNISHD_SECONDARY_STORAGE.