Merge PR #25619 into master

* refs/pull/25619/head:
	mds: use frag_vec_t for efficient frag storage
	mds: do not copy session map accidentally
	mds: use vector for subtree access
	include/frag: use more efficient small vectors
	StackStringStream: add strv method short-cut
	include: add small_vector encode/decode/<<
	mds: scope using directives

Reviewed-by: Venky Shankar <vshankar@redhat.com>

diff --cc src/mds/MDCache.cc
Simple merge

diff --cc src/mds/MDSRank.cc
Simple merge

diff --cc src/mds/Server.cc
Simple merge

diff --cc src/mds/SessionMap.h
Simple merge