os/bluestore: create the tail when first set FLAG_OMAP

os/bluestore: create the tail when first set FLAG_OMAP

the omap iterator when listing omap use the tail of '~', when the iterator moved to the last key of the omapswe wanted, we will try to call extra next(), usually this will be another object's omap header(with '-'). IF there are some deleted key or tombstones, rocksdb will fall in the loop of FindNextUserEntryInternal until find a valid key, so it will travels all dead key in mid and read the sst file heavily.

Signed-off-by: Tao Ning <ningtao@sangfor.com.cn>