Move event hashing out of balloon internals

[jllucas]

Move hashing from balloon internals (balloon.go) to Raft (raft.go), just before inserting the command to the WriteAheadLog.

Others:

• AddEventCommand and AddEventBulkCommand modified.

[jllucas]

BenchmarkRaftAdd with b.N = 2000000

HASHING OUTSIDE BALLOON

event := utilrand.Bytes(128)

| => du -sh /var/tmp/raft-test/node1/raft/wal/*

176M /var/tmp/raft-test/node1/raft/wal/000003.log
52K /var/tmp/raft-test/node1/raft/wal/LOG

event := utilrand.Bytes(2048)

| => du -sh /var/tmp/raft-test/node1/raft/wal/*

176M /var/tmp/raft-test/node1/raft/wal/000003.log
52K /var/tmp/raft-test/node1/raft/wal/LOG

HASHING INSIDE BALLOON (MASTER BRANCH)

event := utilrand.Bytes(128)

| => du -sh /var/tmp/raft-test/node1/raft/wal/*

224M /var/tmp/raft-test/node1/raft/wal/000003.log
145M /var/tmp/raft-test/node1/raft/wal/000011.log
52K /var/tmp/raft-test/node1/raft/wal/LOG

event := utilrand.Bytes(2048)

| => du -sh /var/tmp/raft-test/node1/raft/wal/*

256M /var/tmp/raft-test/node1/raft/wal/000003.log
257M /var/tmp/raft-test/node1/raft/wal/000011.log
257M /var/tmp/raft-test/node1/raft/wal/000012.log
257M /var/tmp/raft-test/node1/raft/wal/000014.log
257M /var/tmp/raft-test/node1/raft/wal/000015.log
257M /var/tmp/raft-test/node1/raft/wal/000017.log
257M /var/tmp/raft-test/node1/raft/wal/000018.log
256M /var/tmp/raft-test/node1/raft/wal/000020.log
256M /var/tmp/raft-test/node1/raft/wal/000021.log
256M /var/tmp/raft-test/node1/raft/wal/000023.log
257M /var/tmp/raft-test/node1/raft/wal/000024.log
257M /var/tmp/raft-test/node1/raft/wal/000026.log
256M /var/tmp/raft-test/node1/raft/wal/000027.log
257M /var/tmp/raft-test/node1/raft/wal/000029.log
257M /var/tmp/raft-test/node1/raft/wal/000030.log
256M /var/tmp/raft-test/node1/raft/wal/000032.log
97M /var/tmp/raft-test/node1/raft/wal/000033.log
496M /var/tmp/raft-test/node1/raft/wal/000034.sst
113M /var/tmp/raft-test/node1/raft/wal/000035.sst
132K /var/tmp/raft-test/node1/raft/wal/LOG