bitbake: lib/bb/main.py: Shutdown cooker when server isn't foreground

The cooker is added to the BBServer and then detached creating
a copy of the cooker in the process, if the server isn't in
foreground it cause fd leaks on inotify this can be see using
many instances of tinfoil.

Example:

from bb.tinfoil import Tinfoil
while True:
    with Tinfoil() as tinfoil:
        input("Pre\n")
        tinfoil.prepare(config_only=True)
        input("Post\n")

[YOCTO #10873]

(Bitbake rev: 2ada2efb057a291eb34c66d975eb339232137956)

Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/main.py

@@ -367,6 +367,7 @@ def start_server(servermodule, configParams, configuration, features):
         raise
     if not configParams.foreground:
         server.detach()
+        cooker.shutdown()
     cooker.lock.close()
     return server