Make ConfigWriter internal lock re-entrant
Make ConfigWriter internal lock re-entrant

The ConfigWriter now allows nested locking calls, and acquires
(releases) the config lock from WConfd only the first (last) time.

This is needed for blocks that acquire a lock on the configuration and
use synchronized configuration calls inside. While this should be
avoided in general, it is required for some special circumstances.

Also add a warning that ConfigWriter isn't thread-safe any more.

Signed-off-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>
Reviewed-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>

ganeti-github-testing/ganeti-test-1,andir/ganeti,grnet/snf-ganeti,dimara/ganeti,mbakke/ganeti,mbakke/ganeti,grnet/snf-ganeti,yiannist/ganeti,yiannist/ganeti,ganeti/ganeti,bitemyapp/ganeti,bitemyapp/ganeti,ganeti/ganeti,dimara/ganeti,andir/ganeti,ganeti/ganeti,andir/ganeti,onponomarev/ganeti,apyrgio/ganeti,leshchevds/ganeti,mbakke/ganeti,apyrgio/ganeti,leshchevds/ganeti,onponomarev/ganeti,leshchevds/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1