Continuation: rework interfaces into a "serial" interface

Prepare to add "parallel" callbacks as well, and handle them reasonably.

Signed-off-by: Greg Farnum <greg@inktank.com>
CInode: update to match the Continuation's serial interface changes.

Signed-off-by: Greg Farnum <greg@inktank.com>
Continuation: stupid fixups
Continuation: add parallel mode

Signed-off-by: Greg Farnum <greg@inktank.com>
CInode: allow validate_disk_state to pass stuff with old on-disk versions

Signed-off-by: Greg Farnum <greg@inktank.com>
MDS: MDRequestImpl: provide filepath/filepath2 substitute for MClientRequest

Use this passthru in the Server path locking functions so that we can get
locks or auth pins without an associated MClientRequest.

Signed-off-by: Greg Farnum <greg@inktank.com>
wire up scrub command a little bit more (not done yet at all)
move [performed|passed]_validation into CInode::validated_data struct
CInode: implement dump_validation_results()

Signed-off-by: Greg Farnum <greg@inktank.com>
MDS: MDCache: finish scrub_dentry(), hook up via admin socket

Signed-off-by: Greg Farnum <greg@inktank.com>
CInode: fixup is_dirty() check

Signed-off-by: Greg Farnum <greg@inktank.com>
CInode: Continuation: begin() from the Continuation, not the wrapper function

Signed-off-by: Greg Farnum <greg@inktank.com>
CInode: _validate_disk_state: fixups and debug output

Signed-off-by: Greg Farnum <greg@inktank.com>
MDS: inode_backtrace_t::compare

Introduce a compare() function which checks two backtraces and indicates
if they're equivalent (or divergent!) and the relative freshness.

Signed-off-by: Greg Farnum <greg@inktank.com>
CInode: make use of memory_backtrace_t::compare to be less buggy

Signed-off-by: Greg Farnum <greg@inktank.com>
FIXUP "CInode: implement dump_validation_results()"