HANDLER READ doesn't upgrade metadata lock from S to SR

This problem was fixed along with MySQL WL#6671 "Improve scalability by not using thr_lock.c locks for InnoDB tables".

Change code for HANDLER READ statements to upgrade S metadata lock to
SR metadata lock for the duration of read. This allows us properly
isolate HANDLER READ from LOCK TABLES WRITE and makes metadata locking
for these statements consistent with locking for other DML.