unique transactions numbers for solver files

[stengel]

In order to make sure a solver solves the right game if several instances of GTE run simultaneously, need unique transaction numbers for each game that are appended to the file names for the solvers. They should probably be generated centrally on the server itself: i.e. first a request for a unique transaction number (generated "atomically" so it cannot be generated twice), then use of that transaction number to generate the filename (which is all the solver cares about). For several instances of GTE (including local ones), the transaction number should be from a separate block, e.g. 1000000-1999999 for GTE in Liverpool, 2000000-2999999 for LSE, 3000000-3999999 AWS. Maybe none (0 prefix) for testing and local, to keep them small? Maybe 1,000,000 max instances is a bit exaggerated, 100,000 (5 digits) should suffice.