LU-10418 flr: revise lease API

LU-10418 flr: revise lease API

Introduce two lease API llapi_lease_{acquire,release}() to replace
confusing llapi_lease_{get,put}(). Rename llapi_lease_ext_get() to
llapi_lease_set(). Implement new mirror_extend_layout() to separate
lfs_migrate().

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I88ab1f7f27aa81c44418aecf31c9e89e494fc01e
Reviewed-on: https://review.whamcloud.com/30363
Tested-by: Jenkins
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>