* Add netstandard2.0 target to SE.Redis projects

Two new conditional compilation constants are introduced:

* FEATURE_PERFCOUNTER - supported in net45 and net46
* FEATURE_THREADPOOL - supported in net45, net46, and netstandard2.0

* Add netcoreapp2.0 target framework to test projects

* Replace CORE_CLR conditional compilation symbol

with built-in NETSTANDARD1_5 or NETCOREAPP1_0 since now netstandard2.0
is also supported.

* Remove unnecessary conditional compilation constants