vinyl: add per-index disk write statistics

This patch adds the following counters to the disk section index.info:

  dump                  # dump statistics:
    count               #   number of invocations
    in                  #   number of input statements
      rows
      bytes
    out                 #   number of output statements
      rows
      bytes

  compact               # compaction statistics:
    count               #   number of invocations
    in                  #   number of input statements
      rows
      bytes
    out                 #   number of output statements
      rows
      bytes

Needed for #1662