vinyl: add per-index cache statistics

This patch adds the cache section to index.info with the following
counters in it:

  cache
    rows                # number of tuples in the cache
    bytes               # cache memory size
    lookup              # lookups in the cache
    get                 # reads from the cache
      rows
      bytes
    put                 # write to the cache
      rows
      bytes
    invalidate          # overwrites in the cache
      rows
      bytes
    evict               # evictions due to memory quota
      rows
      bytes

Needed for #1662