Adds a -R option to Snabb Switch which starts an interactive
Read-Eval-Print-Loop.
[bench_env] Use `run_nfv' where applicable.
[bench_env] Use Bash's `${VAR?}' syntax before using user
defined environment variables.
[bench_env] Pass nfv config to `run_nfv' in the individual
scripts.
Performance tweaks in core/timer.lua

The definition of call_timers() in core/timer.lua is moved out of
run_to_time() to avoid generation of the non-compileable bytecode
UCLO (bytecode 51 in the current version of LuaJIT).

Wrapping the call to ffi.C.get_time_ns() in run() with tonumber()
avoids the allocation of a cdata object that is difficult for the sink
optimizer to remove.
Merge pull request #337 from alexandergall/perf-tweaks-2

Performance tweaks in core/timer.lua
Merge pull request #334 from eugeneia/repl

Adds a -R (REPL) option to Snabb Switch
Merge pull request #328 from eugeneia/bench_env-regression

Fix bench_env regressions and improve error reports