Cache validation result of individual uploaded files

[frosch123]

Validation of big files (I tried a NewGRF with 850 MB) takes several seconds (probably computing MD5). The validation is repeated for every PUT/GET, even when only updating meta-data like description.

Please cache the validation result of uploaded files, if they do not change.

[TrueBrain]

This is not completely trivial to implement, because of the many places the session object can be modified.

I suggest to make this change while also refactoring some of the code, so that session dict becomes an object. After that it is a lot easier to invalidate the cache. And .. it makes the code a lot better :D

As this hurts performance, increasing priority (during those few seconds, the API stalls for everyone, as this is disk I/O)