MXFileStore: Store events JSON strings instead of doing a lot of things in [MXEvent encodeWithCoder] about matrix-ios-sdk

[manuroe]

For reference, here are the figures for loading and storing on a 5s the 13184 state events of Matrix HQ (https://riot.im/develop/#/room/#matrix:matrix.org):

Loading (ms)	Storing (ms)
1000	2500

Loading is currently done with [NSKeyedUnarchiver unarchiveObjectWithFile:]
Storing with [NSKeyedArchiver archiveRootObject:toFile:]

[manuroe]

The AFNetworking JSON deserialiser is based on NSJSONSerialization which is a closed black box which converts a whole NSData object to an NSDictionary. We have no control during the parsing. NSJSONSerialization has no delegates. It does not send any notifications on incremental parsing. So we cannot get intermediate JSON strings :/

I gave a try to SBJson5 (http://www.sbjson.org/). It offers more control but it is much more slower.
Parsing the JSON of the an initial sync response gives:

\	Parsing time (ms)
NSJSONSerialization	614
SBJSON	4979