Disable bloom filter sync in Preview- and SearchCommunity.

The new property dispersy_enable_bloom_filter_sync is used to:
1. stop the community from sending bloom filters to sync, and
2. have the community accept incoming messages of any global time
   value.
Add search bloom even though allow_sync parameter is False.
Merge remote-tracking branch 'remotes/upstream/devel' into enable-sync
Merge remote-tracking branch 'remotes/upstream/devel' into enable-sync
Give more specific reason when search fails.
Merge pull request #217 from boudewijn-tribler/enable-sync

Disable bloom filter sync in Preview- and SearchCommunity.