Remove memmap dependency

[Ilya Baryshnikov]

We had a memory leak related to working with too many open files simultaneously, because phase1-coordinator and phase1-cli rely on memmap. Also, verifier and contributor require to write the challenge on disk before running the computation. And file system calls often return errors. Having the ability to do the computations in memory rather than on open files will reduce the possibility of runtime errors, as well as the possibility of memory leaks related to holding the files open for too long. We can safely replace the memory map with fs read/write operations.