idea: add a latency / bandwidth emulator option

[ThomasWaldmann]

it's always a bit tricky to estimate or test performance of remote backends:

• you need to set up accounts, deal with passwords
• network performance might be fluctuating
• difficult to test on slower or faster networks than you actually have

idea:

we could add some options to the posixfs backend:

• latency: it would just add that latency to any backend call
• bandwidth: it could compute how long a load/store would take according to the size of the data and add that on top of the latency

[ThomasWaldmann]

Related: guess a collection of typical latency/bandwidth values for misc. internet connections would be interesting.

[ThomasWaldmann]

Initially I thought about adding this to posixfs backend, but I guess it is simpler and more versatile to add it to Store higher level API.

When using it, one would usually use the store with a posixfs backend, so that one can assume that the posixfs latency is negligible and the bandwidth is very high, so both can be ignored and that primarily the configured latency and bandwidth values determine the timing.

That way, it could be also used with other backends like sftp (or any future backend), to make them slower. E.g. to simulate an internet sftp server while actually using a LAN or localhost sftp server.