`MigrationMode` was kind of defined in the context of an internal mode
used by `migrateBookmark(...)`.
But now, with `getBestTargetBookId`, it is broken.
This commit fix that and the associated implementation.
Now `UPGRADE_ONLY` will make `getBestTargetBookId` return only newer books.
and `ALLOW_DOWNGRADE` will return older books only if current book is
invalid.
As we now always use Library through a shared_ptr, we can make `Library`
not copiable and not movable.
So we can move back the data in `Library`, we don't care about move
constructor.
The why of this mutex is in `Library` is a bit complex.
It has been introduced in c2927ce when there was only `Library` and no
`std::unique_ptr<Impl>`.
As introducing the mutex imply implementing the move constructor, we have
split all data in `LibraryBase` (and keep a default move constructor here)
and add the mutex in `Library` (and implement a simple move constructor).
Later, in 090c2fd, we have move the `LibraryBase` to `Library::Impl`
(which should have been `Library::Data`).
So at the end, `Library::Impl` is never moved. We can move the `mutex` in
it and still simply implement move constructor for `Library`.
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.
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.
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, ...)
Library became thread-safe with the exception of `getBookById()`
and `getBookByPath()` methods - thread safety in those accessors is
rendered meaningless by their return type (they return a reference
to a book which can be removed any time later by another thread).
Introducing a mutex in `Library` necessitates manually implementing the
move constructor and assignment operator. It's better to still delegate
that work to the compiler to eliminate any possibility of bugs when new
data members are added to `Library`. The trick is to move the data into
an auxiliary class `LibraryBase` and derive `Library` from it.
The search text in the catalog query is interpreted as partial by
default, but partial query mode can be disabled in C++. The latter
possibility is not exposed via the /catalog/search kiwix-serve endpoint,
though.
