Add lmdb backend options.

[Heidistein]

This PR adds support for the lmdb backend of pdns. I have conformed to the "params" method of supplying configuration options, to let it blend in with the rest.

[ju5t]

@Heidistein thanks for the PR! Could you add the new variables to the README as well?

[Heidistein]

Ah, yes I'll see to that.
Also, the backend has a few more options. I don't really feel they should all be added to the init.pp. (this is my opinion).
Perhaps it's an idea to move the optional variables to the actual class. This way a user can change them per hiera on the actual class.
I understand this is different than the rest of the class, using the params pattern. It's up to you. Or perhaps a hybrid, keep thos variables as they are now, add the others in the backend class. Or just leave it like it is. Or change it all, and get rid of the parameters alltogether. (that will take more time)

For this PR, i'll add the opts to the readme.

Edit: I wrote this before coffee, I made some thinking errors. ignore the params part, that is not easy/practical to change too much.