client: unify cap flush and snapcap flush

client: unify cap flush and snapcap flush

This patch includes following changes
- assign flush tid to snapcap flush
- remove session's flushing_capsnaps list. add inode with snapcap
  flushes to session's flushing_caps list instead.
- when reconnecting to MDS, re-send one inode's snapcap flushes and
  cap flushes at the same time.

Signed-off-by: Yan, Zheng <zyan@redhat.com>