tablet: use proper locking when starting transaction

This is a follow-up to fd7a40367a81555230eac62a1a8d1467433ed100.

Before this, TxnParticipantTest.TestConcurrentTransactions failed 2/20
times in ASAN mode. With this patch, it passed 200/200 times.

Change-Id: I85d39f0ec4322a9e3c834bc952c97a5112f764eb
Reviewed-on: http://gerrit.cloudera.org:8080/16759
Reviewed-by: Alexey Serbin <aserbin@cloudera.com>
Tested-by: Kudu Jenkins
1 file changed