Plugin causes subsequent builds to fail by trying to open CacheClient.log log again

[AArnott]

The first build is fine, except that the CacheClient.log file remains locked, as evident by failed attempts to delete it, and subsequent attempts to build failing with:

MSBUILD : warning : BeginBuildAsync failed after 14 ms: System.IO.IOException: The process cannot access the file 'CacheClient.log' because it is being used by another process.

[dfederm]

Note to self from investigation: We dispose the BXL Logger object, but that apparently doesn't dispose the underlying ILog instances (FileLog in our case), leading to the log file handle not being closed.