Support for (parallel) append operations

Currently, open with O_APPEND is not supported as per gkfs_open(). Nevertheless, if this part of the code is removed, append is actually functional (if the MergeOperand is fixed). This allows sequential append operations but not parallel as there is a race condition between open, update_size, and write operations with parallel append operations.

The whole append code path needs to be reevaluated and the update size RPC handler needs to be fixed as outlined in #12 (closed) .