add test reproducing crash when concurrently calling Manager.getDatabase

Calling Manager.getDatabase() twice concurrently can cause:

java.lang.NullPointerException
            at com.couchbase.lite.android.AndroidSQLiteStorageEngine.execSQL(AndroidSQLiteStorageEngine.java:95)
            at com.couchbase.lite.Database.initialize(Database.java:888)
            at com.couchbase.lite.Database.open(Database.java:918)
            at com.couchbase.lite.Manager.getDatabase(Manager.java:229)
Merge pull request #297 from mjq/get_database_crash

Add test reproducing crash when concurrently calling Manager.getDatabase