FileTarget - Skip FileSystemWatcher unless ConcurrentWrites=true

[snakefoot]

Trying to resolve #3321

When running on a non-Windows platform with ConcurrentWrites=false, then just use simple polling of File.Exists() before every file-write (throttled to only be done once every 1 sec).

The polling on non-windows-platform can be disabled by KeepFileOpen=false or ConcurrentWrites=true or EnableFileDelete=false.

PR #1878 introduced the use of FileSystemWatcher to improve KeepFileOpen=true + EnableFileDelete=true on Windows. But it was a mixed gift for other platforms.

[codecov-io]

Codecov Report

Merging #3322 into dev will decrease coverage by <1%.
The diff coverage is 38%.

@@          Coverage Diff           @@
##             dev   #3322    +/-   ##
======================================
- Coverage     80%     80%   -<1%     
======================================
  Files        356     356            
  Lines      28100   28141    +41     
  Branches    3743    3749     +6     
======================================
+ Hits       22509   22529    +20     
- Misses      4509    4528    +19     
- Partials    1082    1084     +2

[304NotModified]

Is this one safe for 4.6.3? Or should it be 4.7/5.0?

[snakefoot]

I think it is safe for NLog 4.6.3

[304NotModified]

also merged, thanks again!