add cache usage to cache.stats()

Right now it is PITA to determine real cache usage because LMDB file size does not reflect space freed by garbage collector etc.

I propose to add new item usage to table returned by cache.stats(). It should be float number <0,100> %.

It's value should be computed like Number of pages used/Max pages. Command line equivalent for testing purposes is:

$ mdb_stat -e <cache_path>
Environment Info
  Map address: (nil)
  Map size: 104857600
  Page size: 4096
  Max pages: 25600
  Number of pages used: 15
  Last transaction ID: 766
  Max readers: 126
  Number of readers used: 0