fix: eliminate file intermediary in translate algebra (#2267)

Previously, `rdflib.plugins.sparql.algebra.translateAlgebra()` maintained state via a file, with a fixed filename `query.txt`.  With this change, use of that file is eliminated; state is now maintained in memory so that multiple concurrent `translateAlgebra()` calls, for example, should no longer interfere with each other.

The change is accomplished with no change to the client interface.  Basically, the actual functionality has been moved into a class, which is instantiated and used as needed (once per call to `algrebra.translateAlgebra()`).