chatview: major efficiency issues in updateTimestamps

updateTimestamps() is affected by several design issues with a large drawbacks on performances:

1. This function goes through all timestamp entries and is called by addOrUpdateMessage(). This means that this function is called each and every time a message is displayed ! This is basically O(n²) algorithm. For instance, displaying 50 messages triggers 1275 timestamp updates.
2. This function handles both text and position/style updates. The is probably done in order to simulate some kind of responsive behaviour. This should be done by CSS, not JS.