common: add Formatter class

common: add Formatter class

This is based on the RGW class, but

 - uses a stringstream
 - has an additional dump_stream() method that gives you a usable ostream
 - handles object keys properly

We should merge these implementations.

Not sure if either a stringstream or the raw buffer in the rgw class is
ideal.  Probably output should accumulate on a bufferlist so we can avoid
ever reallocating for big dumps.

Signed-off-by: Sage Weil <sage@newdream.net>