Emacs: Add address completion mechanism implemented in elisp

Emacs: Add address completion mechanism implemented in elisp

Currently, notmuch has an address completion mechanism that requires
external command to provide completion candidates. This patch adds a
completion mechanism inspired by https://github.com/tjim/nevermore,
which is implemented in Emacs lisp only.

The preexisting address completion mechanism, activated by pressing TAB
on To/Cc lines, is extended to use the new mechanism when no external
command is configured, i.e. when notmuch-address-command to nil, which
is the new default.

The core of the new mechanism is the function notmuch-address-harvest,
which collects the completion candidates from the notmuch database and
stores them in notmuch-address-completions variable. The address
harvesting can run either synchronously (same as with the previous
mechanism) or asynchronously. When the user presses TAB for the first
time, synchronous harvesting limited to user entered text is performed.
If the entered text is reasonably long, this operation is relatively
fast. Then, asynchronous harvesting over the full database is triggered.
This operation may take long time (minutes on rotating disk). After it
finishes, no harvesting is normally performed again and subsequent
completion requests use the harvested data cached in memory. Completion
cache is updated after 24 hours.

Note that the change of the notmuch-address-command default value
may *BREAK EXISTING SETUPS* when the user used external command named
"notmuch-addresses", i.e. the previous default. The result will be that
the user will use the new mechanism instead of the his command. I
believe that many users may not even recognize this because the new
mechanism works the same as
http://commonmeasure.org/~jkr/git/notmuch_addresses.git and perhaps also
as other commands suggested at
http://notmuchmail.org/emacstips/#address_completion.

---
Changes from v3:
- Implemented both synchronous and asynchronous harvesting. The
  synchronous implementation that uses faster "filtered" query is used
  until the full asynchronous harvesting finishes.
- Added automatic refresh of completion cache every 24 hours.

Changes from v2:
- Updated Makefile.local to not conflict with current master

Changes from v1:
- Use of notmuch-parser.el instead of the custom parser in the
  original code. The notmuch parser is slightly faster.
- Use of functions in notmuch-query.el instead of functions in the
  original code with almost the same functionality.
- Integrated with existing completion mechanism in notmuch.
- notmuch-company.el was moved from emacs/contrib to emacs and
  no-byte-compile directive was added to it.
- Aligned with notmuch naming conventions.
- Documented bugs found in notmuch-company.el