IO: Move mutex from per-entry to per-transaction based.

Mutex un/lock could be expensive, especially if this is done on a
per-entry basis. Changed to lock for the entirety of one transaction. In
the future, we should check to see if there is a need to lock at all.

Bug: 186164633
Test: CTS, GCA
Signed-off-by: Edmond Chung <edmondchung@google.com>
Change-Id: I05370ede8f34ac5bfdcd6ca7cd3f5c8b18058d36