Syntax:
KEY[>=,<=,>,<,=,:]VALUE
VALUE is expected to be json dump except when the delimiter is ":"
With ":" delimiter, accepted special words are "max" and "min".

Selection:
For the sake of lisibility, the shadir entries are preprocessed in order
to eliminate invalid metadata. This is slower but the metadata is
exepected to be small as each entry may require a database access on
the server shacache side.