Reduce the default revprop pack file size to 4kByte.

This still gives us most of the advantage (typ. 10x reduction in file
count and wasted OS file system clusters, data mainly co-located on disk
due to the packing process itself) but significantly reduces pack file
header parsing overhead.

* subversion/libsvn_fs_fs/fs_fs.c
  (read_config): Reduce the defaults by a factor of 16.
  (write_config): Update the corresponding sections in the fsfs.conf
                  template.  Remove references to the revprop caching
                  feature as it is currently unavailable.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1634872 13f79535-47bb-0310-9956-ffa450edef68