Manually invoking the GC leads to very bad performance if you read many files

[steffenhaug]

I have a workload where i read thousands of Excel files, and using openxlsx without do-syntax is several orders of magnitudes faster, because over 99% of the time is spent GCing when I use do-syntax.

I'm fine with manually closing the file, but it would be nice if it was documented that using do-syntax manually invokes the GC, or if there is another way to solve the bug on Windoes that manually invoking the GC is suposed to fix.

[TimG1964]

PR #280 will remove all explicit GC.gc calls (if accepted).