[MC-2082] Implement new event storage

[nishant-clevertap]

Overview

We need to implement new events storage that is persisted across sessions and on user switches. We have used SQLite to store data based on our requirement and it is also light weighted.

Implementation

Created class CTEventDatabase which will handle storing of user events data in table CTUserEventLogs. The schema used is as - eventName, count, firstTs, lastTs, deviceID where eventName and deviceID combined is used as primary key. We have added methods to create table, insert data, update data, check if event exists or not, and to get count, first and last time stamp of event for particular deviceID. Added unit tests also for these methods.

[akashvercetti]

@nishant-clevertap i have some suggestions.
1.createTable hasn't been called anywhere, should we call it in initWithConfig at the start?
2. should we use database schema version for future db migrations?
3. for the insert and update queries, can we use sqlite3_bind_text and sqlite3_bind_int instead of NSString.
4. can we check if (!_eventDatabase) before every operation begins for safety
5. can dispatch_sync cause deadlocks or hangs? should we use something else?

@nzagorchev any suggestions?