Use a single redis connection per process

[mishaschwartz]

In order to reduce the number of total redis connections used by the autotester:

• use a single redis connection by each of the API (client) processes. This is declared as a global variable
• use the same redis connection used by the rq job in each worker process
• use a single redis connection in the installer script
• use a single redis connection in the start_stop script

Note that in the API, two types of connection were previously required: one that decoded all responses and another that did not. In order to use a single connection, the code that assumed that responses are decoded are updated to handle decoding as needed.