Fixes issues with subscription vc self references and setup.

Moves child vc setup out of init.
Uses weak references, especially in blocks, to avoid a circular retain cycle.
Set shouldRefreshTableViewPreservingOffset flag before calling service.
Updates when willRefreshTableViewPreservingOffset flag is set.

Fixes an issue where cell heights were estimated and not actual when refreshing.
Move flag toggle later in method (code style). Whitespace changes.
Document new methods
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/reader-performance-improvements
Adds check for existing context before syncing.
Merge pull request #2960 from wordpress-mobile/issues/reader-performance-improvements

Issues/reader performance improvements