Add bayes_expiry.conf to provide ability to set custom settings

[dragoangel]

No description provided.

[moisseev]

In my opinion, there's no necessity to redefine module defaults. Instead, I suggest commenting parameters out and leaving them as examples.

Short comments on their meanings would be useful as well. ;)

[dragoangel]

I just do it same way I see it in other modules, it has a place in other conf. Also maybe instead of adding comments I better expand rspamd docs?

[dragoangel]

Removed settings duplication. Will create PR to docs separately

[dragoangel]

@moisseev rspamd/rspamd.com#748 please check, if this is okay, can you approve\merge both PRs please?

[dragoangel]

@vstakhov can we merge this please to allow users to adjust bayes expiry settings? I also found out that cluster_nodes is obsolite but wasn't removed after efe8fd4 PR from other parts of the script. I think we can remove them to no create confusion from:

• https://github.com/rspamd/rspamd/blob/master/src/plugins/lua/bayes_expiry.lua#L36
• https://github.com/rspamd/rspamd/blob/master/src/plugins/lua/bayes_expiry.lua#L170-L179

I not sure, but I have feeling that before switching to LUA based scripts bayes expiry could process mail with default settings, but after switching to LUA - default count of 1000 is not enough, at least this was the case in my case. I had to increase count in 15 times, but by default we can think about setting 2000 as example?