Add a new opcode timestamp field

Since the current start_timestamp opcode attribute refers to the inital
start time, before locks are acquired, it's not useful to determine the
actual execution order of two opcodes/jobs competing for the same lock.

This patch adds a new field, exec_timestamp, that is updated when the
opcode moves from OP_STATUS_WAITLOCK to OP_STATUS_RUNNING, thus allowing
a clear view of the execution history. The new field is visible in the
job output via the 'opexec' field.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>