Server should use random names for TMP directories to avoid problems when running multiple instances

In order to run multiple instance of the Appium server, once might assume from the documentation that setting a unique port number is sufficient. It turns out this is not the case.

The reason is that if no TMP directory is explicitly set (on the Mac), then /tmp is used. This causes problems since the instruments trace file, the run results, and the instruments_sockets file are being shared between instances and this causes unexpected results and lost log data. Manually setting the TMP dir with each instance alleviates the problems.

A random name should be used for each new server instance.