lvmetad: add invalidation method

lvmetad: add invalidation method

Add the ability to invalidate global or individual VG metadata.
The invalid state is returned to lvm commands along with the metadata.
This allows lvm commands to detect stale metadata from the cache and
reread the latest metadata from disk (in a subsequent patch.)

These changes do not change the protocol or compatibility between
lvm commands and lvmetad.

Global information
------------------

Global information refers to metadata that is not isolated
to a single VG , e.g. the list of vg names, or the list of pvs.

When an external system, e.g. a locking system, detects that global
information has been changed from another host (e.g. a new vg has been
created) it sends lvmetad the message: set_global_info: global_invalid=1.
lvmetad sets the global invalid flag to indicate that its cached data is
stale.

When lvm commands request information from lvmetad, lvmetad returns the
cached information, along with an additional top-level config node called
"global_invalid".  This new info tells the lvm command that the cached
information is stale.

When an lvm command sees global_invalid from lvmated, it knows it should
rescan devices and update lvmetad with the latest information.  When this
is complete, it sends lvmetad the message: set_global_info:
global_invalid=0, and lvmetad clears the global invalid flag.  Further lvm
commands will use the lvmetad cache until it is invalidated again.

The most common commands that cause global invalidation are vgcreate and
vgextend.  These are uncommon compared to commands that report global
information, e.g. vgs.  So, the percentage of lvmetad replies containing
global_invalid should be very small.

VG information
--------------

VG information refers to metadata that is isolated to a single VG,
e.g. an LV or the size of an LV.

When an external system determines that VG information has been changed
from another host (e.g. an lvcreate or lvresize), it sends lvmetad the
message: set_vg_info: uuid=X version=N.  X is the VG uuid, and N is the
latest VG seqno that was written.  lvmetad checks the seqno of its cached
VG, and if the version from the message is newer, it sets an invalid flag
for the cached VG.  The invalid flag, along with the newer seqno are saved
in a new vg_info struct.

When lvm commands request VG metadata from lvmetad, lvmetad includes the
invalid flag along with the VG metadata.  The lvm command checks for this
flag, and rereads the VG from disk if set.  The VG read from disk is sent
to lvmetad.  lvmetad sees that the seqno in the new version matches the
seqno from the last set_vg_info message, and clears the vg invalid flag.
Further lvm commands will use the VG metadata from lvmetad until it is
next invalidated.