crimson/mon/MonClient: fix use-after-free in run_command

crimson/mon/MonClient: fix use-after-free in run_command

run_command() captured a reference to the just-emplaced mon_commands
element into the send_message() continuation. mon_commands is a
std::vector, so a concurrent run_command() could reallocate it and
invalidate the reference before the continuation ran, dereferencing
freed memory.

No caller issues concurrent commands today, but the pattern is unsafe.

Take the result future before issuing the message, then coroutinize
the method so it is awaited directly instead of being captured into a
continuation lambda.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>