global: Fix spelling mistakes in comments Original work by @andreasschulze and @jsoref

lib-storage: If dovecot.index.thread corruption is noticed, delete the file.

Rewrote thread indexing code. It's a lot simpler and takes less disk space. We no longer try to keep a hash table and the entire thread tree stored on disk. Instead we keep a simple Message-ID string (actually just "uid, ref#" pointer) -> unique index number mapping on disk, read it to memory and use it to build the thread tree. After the initial build the thread tree is still updated incrementally. --HG-- branch : HEAD