Shared pointer should be used where appropriate

[kelson42]

See kiwix/kiwix-android#3467

[kelson42]

@veloman-yunkan @mgautierfr Reading #991 (review), it seems to me there is a discrepency between both of you on (1) the nature of the problem (2) the path to follow to improve the situation. I have no optinion at all, but I would prefer to have you reaching a kind of of agreement on these two points before continuing PR implementation.

[mgautierfr]

Here a proposition:

• Move html_dumper, libxml_dumper and opds_dumper behind a free function (implementation will not be changed, but classes will not be part of the public api).
• Make server take a shared_ptr to library and namemapper.
• Keep the proposition in Use std::shared_ptr instead of raw pointer. #991 to create the library only on the heap through a shared_ptr (?)
• If previous point is not done, introduce a free function as proposed here to be able to create a not owning shared ptr (internal only ?) for case when user want to create a a server on library

[veloman-yunkan]

Here a proposition:

• Move html_dumper, libxml_dumper and opds_dumper behind a free function (implementation will not be changed, but classes will not be part of the public api).

• Make server take a shared_ptr to library and namemapper.

• Keep the proposition in Use std::shared_ptr instead of raw pointer. #991 to create the library only on the heap through a shared_ptr (?)

• If previous point is not done, introduce a free function as proposed here to be able to create a not owning shared ptr (internal only ?) for case when user want to create a a server on library

I am fine with that