Replicator: add sorted cmd_vg list

Replicator: add sorted cmd_vg list

Introduce struct cmd_vg to store information about needed
volume group name, vgid, flags and the pointer to opened VG.

Keep VGs list in alphabetical order for locking order.

Introduce functions:
cmd_vg_add() add new cmd_vg entry.
cmd_vg_lookup() search cmd_vgs for vg_name.
cmd_vg_read() open VGs in cmd_vgs list.
cmd_vg_release() close VGs in reversed order.