Don't share Lucene::Directory between writers

Multiple IndexWriters can write into the same index directory, but they
shouldn't share the same Directory *object*.
Lucene: use non-MT merge scheduler

Attempt to fix the problem with mysteriously corrupted indexes by using
a simpler, single-threaded merge scheduler.  Hopefully it shouldn't
matter for Poedit's use, because its index is relatively small.

The issue seems to be relevant to older Lucene versions (such as 3.0
from Lucene++) and plagues only Windows.  This fix is the same fix
applied by NetBeans:

https://netbeans.org/bugzilla/show_bug.cgi?id=197515