Decrease redirects memory usage in the writer

[benoit74]

From #1045 (comment), @veloman-yunkan said:

The current implementation maintains a separate collection of not-yet-resolved redirections which takes additional memory and it looks like we can get rid of it.

Also, there is a way to decrease the unjustified memory usage in flows where the count of redirections significantly exceeds the number of real entries (meaning that multiple redirections have to point to the same entry). It can be achieved by sharing the strings of the target path in redirections pointing to the same item.

To be investigated, anything decreasing memory consumption is by design worth to investigate IMHO.

[kelson42]

Implementing this memory usage optimisation around redirects writing, seems to me almost consubstantial, to implement #1001 because the usage of redirects with fragments will lead, for many scrapers, to far more redirects than before.