Display emerge search results incrementally (412471)

This makes emerge --search / --searchdesc display individual search
results as soon as they become available. A search._iter_search method
is split out from the search.execute method, and the search.output
method operates on that. The spinner is now disabled, but the spinner
code remains, in case we later decide to enable it optionally.

X-Gentoo-Bug: 412471
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=412471
Add egencache --update-pkg-desc-index action.

This adds an egencache --update-pkg-desc-index action which generates
a plain-text index of package names, versions, and descriptions. The
index can then be used to optimize emerge --search / --searchdesc
actions.

X-Gentoo-Bug: 525718
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=525718
Add IndexStreamIterator and MultiIterGroupBy.

This IndexStreamIterator class can be used together with the
pkg_desc_index_line_read function to read an index file incrementally
as a stream.

The MultiIterGroupBy class can be used to iterate over multiple
IndexStreamIterator instances at once, incrementally grouping results
for a particular package from multiple indices, while limiting the
amount of any given index that must be in memory at once.

Both of these classes are used by the  IndexedPortdb class in the next
patch of this series.

X-Gentoo-Bug: 525718
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=525718
Add IndexedPortdb class.

The IndexedPortdb class uses pkg_desc_index to optimize searchs for
package names and descriptions. If the package description index is
missing from a particular repository, then all metadata for that
repository is obtained using the normal pordbapi.aux_get method.

This class only implements a subset of portdbapi functionality that is
useful for searching pkg_desc_index incrementally. For this reason,
the cp_all method returns an ordered iterator instead of a list, so
that search results can be displayed incrementally.

X-Gentoo-Bug: 525718
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=525718
Add IndexedVardb class.

Searching of installed packages is optimized to take advantage of
vardbdbapi._aux_cache, which is backed by vdb_metadata.pickle.
This class only implements a subset of vardbapi functionality that is
useful for searching incrementally. For this reason, the cp_all method
returns an ordered iterator instead of a list, so that search results
can be displayed incrementally.

X-Gentoo-Bug: 525718
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=525718
Add emerge --search-index option.

The new emerge --search-index option, which is enabled by default,
causes pkg_desc_index to be used for search optimization. The search
index needs to be regenerated by egencache after changes are made to
a repository (see the --update-pkg-desc-index action).

For users that would like to modify ebuilds in a repository without
running egencache afterwards, emerge --search-index=n can be used to
get non-indexed search. Alternatively, the user could simply remove
the stale index file, in order to disable the search index for a
particular repository.

In order to conserve memory, indices are read as streams, and
MultiIterGroupBy is used to group results from IndexedPortdb and
IndexedVardb. Stream-oriented search also makes it possible to
display search results incrementally (for bug #412471).

X-Gentoo-Bug: 525718
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=525718
Log changes between vdb_metadata.pickle updates

This adds add support to generate a vdb_metadata_delta.json file
which tracks package merges / unmerges that occur between updates to
vdb_metadata.pickle. IndexedVardb can use the delta together with
vdb_metadata.pickle to reconstruct a complete view of /var/db/pkg,
so that it can avoid expensive listdir calls in /var/db/pkg/*.
Note that vdb_metadata.pickle is only updated periodically, in
order to avoid excessive re-writes of a large file.