Add AccountingSetBytesRead/Written

A relay operator asked for the ability to reset the state file's AccountingBytesReadInInterval and AccountingBytesWrittenInInterval values.

While a workaround is to edit or delete the state file, perhaps command-line arguments AccountingSetRead and AccountingSetWritten could be useful.

See also legacy/trac#15989 (moved) for AccountingRule in and out, as well as the AccountingEnableDirPort override.