Add RWMutex lock into TruncIndex to fix race condition when inserting values

The idx.index array is overwritten when a new value is inserted to the index.
When two containers are created concurrently, their ids are inserted to the
index and one can overwrite the other leaving one of ids missing from the
index. Adding a RWMutex lock around read/write operations fixes this.

Docker-DCO-1.1-Signed-off-by: James Allen <jamesallen0108@gmail.com> (github: jpallen)
Merge pull request #3606 from jpallen/1319-fix-race-condition-in-truncindex

Add RWMutex lock into TruncIndex to fix race condition when inserting values