v0.4.0-yb-5: Add an optional prefix message to every log message

ntroducing log_prefix_func which generates a character string to be used as the prefix of every log message.
SetLogPrefix is used to set this prefix function.

SetLogPrefix will be set to a function that returns a thread local string that identifies its process\service name. This is useful in yb MiniCluster tests which starts several master and tserver services on a single test process. Each thread will store its respective service names in a thread local string that we return via the prefix function.

In xCluster tests where we have two clusters with identical master and tservers we will print a more detailed name [ClusterName serviceName-serviceId] as a prefix.

v0.4.0-hk-5

v0.4.0-hk-5