Unmap index on impending core dump.

Summary:
 - for big indexes, core dumps are prevented due to size limits
 - this unmaps the index when SIGSEGV or SIGABRT is received
 - closes #50

Test Plan:
./bin/mwsd -I ./data/zbl -D Index/;
kill -SIGSEGV $!

Reviewers: cprodescu

Reviewed By: cprodescu

Differential Revision: http://phab.code4fun.de/D81