FDSNWS: add concurrency in assembling request data

[megies]

When assembling requested data, jane is requesting one file at a time sequentially from file system. On slow filesystems (e.g. NFS) with read operation on a single file taking on the order of 0.1-1s this can lead to requests taking a very long time when tens of files need to be accessed.
This might have to be improved by adding concurrency of some sorts, probably something like asyncio with aiofiles could be enough.