Expand query: reimplement the way to do subqueries

For any expand query, the current implementation does all queries in a
single MultiAsyncResp, where the code sends a bunch of requests without
Query parameters. This makes it impossible to invoke efficient expand
handlers, since efficent handlers will only be invoked when a query has
$expand in its parameters. (Delegation only happens when the query
contains query parameters)

To solve it, in this commit, we proposed to send a bunch of requests
**WITH** Query parameters in MultiAsyncResp. This makes
"/redfish/v1/Chassis/chassis?expand=.($levels=2)" be able to invoke
efficient expand handlers that we developed for sensors, which existing
implementation can't do. This decreases latency by nearly 100 times (the
improvement that efficient sensor expand handler provides) on real
hardware which contains 5+ chassis and totally 220+ sensors.

This commit aligns with future $select support well, since the recursive
queries can add $select as part of the query parameters.

With this commit, though we create multiple MultiAsyncResp objects
memory doesn't increase significantly; part of the reason is that we are
not copying Query anymore in MultiAsyncResp.
No out-of-memory issues are found when 4 threads are querying
expand=levels=6 at the service root on a real large hardware which
contains 2+ sockets, 5+ chassis, 220+ sensors, 30+ DIMMs, etc.

Tested:
1. On real hardware, /redfish/v1/Chassis?$expand=.(level=3) is giving
the correct result and invokes efficient sensor Expand handler
2. stress test
```
for i in {1..4};
do
  echo "thread $i"
  wget -qO- 'http://localhost:18080/redfish/v1?$expand=*($levels=6)' > "/tmp/$i.log" &
done

for i in {1..1000};
do
  top -b -n 1 | grep bmcweb >> /tmp/bmcweb_ori.log
  sleep 1
done
```
Results
```
25878  2856 root     R     194m  20%   1  38% /tmp/bmcweb_after
19005  2856 root     R     215m  22%   1  36% /tmp/bmcweb_ori
```

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I0e661db0263f56dd0cab66047a0a5d4fff31b69a

0 files changed, 0 insertions, 0 deletions