libtracker-extract: plug leak
tracker-fts: Make the property list per-thread data

This is global memory, and shared across threads, which doesn't play well
when (un)initializing it, as multiple simultaneous accesses can occur.

Conflicts:
	tracker/src/libtracker-fts/tracker-fts.c
tracker-extract: Fix possible invalid memory read/write

If length is 0, out of bounds data would be read, and then written.
tracker-extract: Invert initialization order

tracker_extract_persistence_invalidate() may already be using indirectly
the priv->persistence hashtable, ensure it's set up before this is called.
libtracker-miner: Reduce TrackerDecorator file cache size

Instead of keeping all items in memory, a small cache is kept of upcoming
files to be processed, querying the database again for files missing the
nie:dataSource when the cache has been flushed. When no further items are
processed, the miner is stopped. Whenever the prioritized rdf:types list
changes, the cache is dropped, just to query again with the new order kept
into account.

This reduces the potential memory usage of tracker-extract, it would
previously would grow linearly with the number of files left to be
inspected, which might bring in some memory pressure on certain scenarios.

On the downside, this obviously makes the decorator a little bit slower,
due to more queries happening (as opposed to a huge one), and at times the
decorator is idle waiting to know what else to do.

Conflicts:
	tracker/src/libtracker-miner/tracker-decorator-internal.h
	tracker/src/libtracker-miner/tracker-decorator.c
tracker-extract: tell what the error really means

The current one is not really intuitive.
tracker-miner-fs: Delete nie:dataSources on update

Decorators should be updating again all relevant information, so ensure
their nie:dataSource is deleted for these to kick in again.

This used to be unnecessary because updated items just went to the
internal decorator queue, without further checks. Now that the decorators
query the database for the next items missing that dataSource, it is
crucial to do this somewhere.
Merge pull request #41 from pvanhoof/decorator-memory-reduction-backport

Decorator memory reduction backport