Optional distributed garbage collection service

A distributed garbage collection service collects garbage for selected hosts (theaters). It can collect distributed acyclic garbage much faster than local collectors do. Furthermore, It can collect distributed cyclic garbage in the selected hosts, while local collectors cannot. To run the distributed garbage collection once, one can use the command as follows:

java -cp salsa<version>.jar:. gc.serverGC.SServerPRID -1 <host1> <host2> ......

To run it every n seconds, use:

java -cp salsa<version>.jar:. java gc.serverGC.SServerPRID <n> <host1> <host2> .....

Note that n must be large enough because the distributed collector is poorly implemented. The performance will be improved in the next version of SALSA.