Add flag to enable listening on IPv6.

 - this will enable IPv6 exclusively (by using MHD_USE_IPv6).
 - in newer versions of libmicrohttpd, MHD_DUAL_STACK should be
 used, which enables listening both IPv4 and IPv6.
Save compressed index directly to disk.

Summary:
 - The previous implementation required the compressed index
 to be built in memory
 - closes #27
 - closes #30

Test Plan:
bin/mws-index -I data/zbl -o data/compressed
bin/mwsd-load -I data/compressed &
scripts/send-qvar-query.sh 9090 0 0

Reviewers: radu.hambasan

Differential Revision: http://phab.code4fun.de/D67
Fix memory leak in LevFormulaDb.