Bump version to librrd-8.dll for MSVC builds

Bump version to librrd-8.dll for MSVC builds

The current version in the filename of the library is 8, e.g.
librrd.so.8 or librrd-8.dll (MinGW-w64 builds), which is based on
LIBVERS from configure.ac. The version of the dll for MSVC builds is
not derived from LIBVERS and has not been updated yet.

- Substitute occurrences of librrd-4 with librrd-8 using:
  git grep -l 'librrd-4' | xargs sed -b -i 's/librrd-4/librrd-8/g'
- rename librrd-4* files to librrd-8* using:
  find . -name 'librrd-4*' \
  -exec bash -c 'file={}; git mv $file ${file/librrd-4/librrd-8}' \;