osd: create tool to extract pg info and pg log from filestore

Initial development for first review

Signed-off-by: David Zafman <david.zafman@inktank.com>
Undo development diagnostics
Get rid of dout() and derr logging by default (enable with --debug)
Eliminate duplicate code in read_log()
Fix --help
Fix comment
Get pg_missing_t in _read_log() - moved rest of code from read_log()
Remove disabled code
Fix dout() macro so it skips logging if this == NULL
Pass ostringstream to _read_log() to handle osd->clog.error() logging
Move read_log() and _read_log()
Merge wip-3890 to wip-3890-review
Rename _read_log()/_read_info() to read_log()/read_info()
Place input args before output args in read_info()/read_log()
   This excludes optional arg passedpg to read_log().
Assert on error return from read_info()

Signed-off-by: David Zafman <david.zafman@inktank.com>