Add traffic simulator using multiple base URLs with timed availability

[tobbee]

It should be possible to specify multiple traffic patterns, where each patten applies to a BaseURL defined in the MPD.

In this case, it is possible to define that one BaseURL should have some down period, while another is up etc.

The expressibility should be something like:

Traffic Patterns for one or more BaseURLs

Specify time interval for loss patterns for one or more BaseURLs with "up (u)", "down (d)", "slow (s)", or "hang (h)" states, like u50d10,u10d50 or d1,u1,u45s10h5 where:

• u50d10,u10d50 is two pairs (two base URLS)
• u50d10 means 50s up time and 10s downtime (60s cycle)
• u10d50 means 10s up time and 50s downtime (60s cycle)

d1,u1,u45s10h5means three base URLS where the first is always down, the second is always up, and the third is up 45s, slow 10s, and hanging 5s every minute

• During a "down (d)" interval, all segment requests will result in a "404 Not Found" response.
• During a "slow (s)" interval, all segment responses are delayed by 2s.
• During a "hang (s)" interval, all segment responses hang for 10s before resulting in 503.

[tobbee]

Brought up in discussion of #91. Something similar, but more limited, was available as baseurl parameter in livesim1.