Move the Library mutex in Library::Impl.

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`.

include/library.h

@@ -178,9 +178,6 @@ class ZimSearcher : public zim::Searcher
  */
 class Library: public std::enable_shared_from_this<Library>
 {
-  // all data fields must be added in LibraryBase
-  mutable std::mutex m_mutex;
-
  public:
   typedef uint64_t Revision;
   typedef std::vector<std::string> BookIdCollection;