Lazily read lines in ts_file_server

This change allows tsung to use a _very_ large file as a variable source; each read will seek to an arbitrary location in the file and perform a read.

It's possible (likely, even) that this change will reduce performance of the variable sourcing feature; I can imagine using a config flag to enable it, perhaps. FWIW I haven't observed a serious degradation of performance in my usage.

Usage remains identical.