cubicweb #13821949 handle index name'size [validation pending]

we may currently go out of bound, depending on the backend, and then loose control on indexes.

prioritynormal
typebug
done in3.23.0
load left0.000
closed by#49aca289134f [sql gen] Control size of index name using an md5 hash
patch[sql gen] Control size of index name using an md5 hash [applied]