Fix iOS threading model

[brody4hire]

The iOS version uses a thread pool to queue and handle SQL batch requests (backgroundExecuteSqlBatch: in SQLitePlugin.m). However, there is effectively only one thread working at a time since executeSql: in SQLitePlugin.m calls executeSqlWithDict:andArgs: in a @synchronized(self) block.

There are two possibilities:

• run all SQL requests from the same thread and build sqlite.c with SQLITE_THREADSAFE=0 (this will work for Windows (8.1) version as well since it has no background threading right now)
• Implement threading with one thread per database (as is already done for Android and WP(7/8) versions)

NOTE: this issue is opened in response to http://ost.io/@litehelpers/Cordova-sqlite-storage/topics/1