[libsocialcache] Improve locking and database cleanup

This commit adds more locking around sensitive code areas, and also
ensures that each derived type is responsible for closing the database
at the appropriate time (depending on the threading being used).
Merge pull request #15 from chriadam/lock

[libsocialcache] Improve locking and database cleanup
Bump version
[libsocialcache] Avoid recreating delegates on model updates.
[libsocialcache] Cache prepared queries.
Merge pull request #16 from adenexter/master

Improve model performance
Bump version
[bugfix] Fixed an issue that generated potential wrong SQL queries

This commit prevents the other keys to be empty during an
UPDATE query.
[feature] Small API change for image downloader

This commit changes the signature of a signal to
transmit the data that were used to download
a specific image.
Merge pull request #17 from SfietKonstantin/sqlbugfix

[bugfix] Fixed an issue that generated potential wrong SQL queries
Merge pull request #18 from SfietKonstantin/dl-api

[feature] Small API change for image downloader
[feature] Enhancements to contacts database

This commit adds some missing methods to the
contacts database, like contacts removal
and cached image management.
Merge pull request #19 from SfietKonstantin/contacts-enhancement

Contacts enhancement
[bugfix] Fixed an issue in updating contact entry

This commit fixes an issue where updating a contact
entry might reset other entries.
Merge pull request #20 from SfietKonstantin/master

[bugfix] Fixed an issue in updating contact entry
Bump version
[feature] Extended image downloader

This commit ads a nicer API for image downloader,
making it easy to customize requests, like signed
requests.
[feature] Be able to store Facebook media URL

This commit provides the capability to cache Facebook
media URL, and expose it in the model.
Merge pull request #21 from SfietKonstantin/abstract-image-downloader

[feature] Extended image downloader
Merge pull request #22 from SfietKonstantin/url

Store media url