Created a generic function multipleQuery which takes:
1. string (representing a comma separated list)
2. param (the value to query on using Xapian).
Category and language query will use this function.
`false` is a pretty bad default value as most user want to track
the real download.
By removing the default value, we force user to make a choice.
We could have change the default value to true but it would have been
a silent API change and we don't want that.
From now on, the `lang` parameter of the /catalog/search,
/catalog/v2/entries, and /catalog/v2/partial_entries endpoints is
interpreted as a comma-separated list of languages.
If we keep a reference to a `Reader` it is better to (share) owning
the reference. Else the reader may be deleted after we create the searcher.
This is especially the case now we are creating the `Reader` at demand
and we don't store it in the library's cache.
We have to reuse the query the user give us to generate the
pagination links.
At search result rendering step we don't have access to the query object.
The best place to know which arguments are used to select books
(and so which arguments to keep in the pagination links) is when we
parse the query to select books.
Fix tests (pagination links) with book selector other than "books.id="
(pattern=jazz&books.query.lang=eng)
libzim's search is not thread safe (mainly because xapian is not).
So we must protect our search objects from multi thread calls.
The best way to do this is to associate a mutex to the `zim::Searcher`
and lock the searcher each time we access object derivated from the
searcher (search, results, iterator, ...)
As we still create a `Reader` in the deprecated code of `Library`,
we need a way to create a reader without raising a deprecated warning.
So we create a another constructor with a dummy argument and we use it.
As the `Entry` is still created by `Reader` we need a way to create a
entry without raising a deprecated warning.
To do so we create a second constructor with a dummy argument.
This second constructor is private and is not marked as deprecated so we
can use it.
