storage_service: Add debug info in gossip state change callback
storage_service: Add debug info in gossip state change callback

Now, we can verify that get_tokens_for(ip) get the correct tokens
through gossip.

Start the seed node first:
$./seastar --listen-address 127.0.0.1 --seed-provider-parameters 127.0.0.1

Then start a normal node:
$./seastar --listen-address 127.0.0.2 --seed-provider-parameters 127.0.0.1

On the second node:

SS::on_join endpoint=127.0.0.1
endpoint=127.0.0.1,
tokens_string=8768e8a2d360bce8;d99d50eae7e514a2;f937ba030e4f1fa2
t=87 68 e8 a2 d3 60 bc e8
t=d9 9d 50 ea e7 e5 14 a2
t=f9 37 ba 03 0e 4f 1f a2

ep=127.0.0.1, eps=EndpointState: HeartBeatState = generation =
1432868747, version = 52, AppStateMap =  { 0 : Value(BOOT,TOKENS,11) }
{ 5 : Value(urchin_1_0,4) }  { 8 : Value(,3) }  { 11 : Value(ms_1_0,1) }
{ 12 : Value(9a6e8c60-05af-11e5-b9b8-000000000000,2) }  { 13 :
Value(8768e8a2d360bce8;d99d50eae7e514a2;f937ba030e4f1fa2,9) }

senseb/scylla,victorbriz/scylla,shaunstanislaus/scylla,duarten/scylla,guiquanz/scylla,linearregression/scylla,aruanruan/scylla,raphaelsc/scylla,phonkee/scylla,asias/scylla,kangkot/scylla,bowlofstew/scylla,gwicke/scylla,raphaelsc/scylla,tempbottle/scylla,glommer/scylla,respu/scylla,acbellini/scylla,justintung/scylla,justintung/scylla,dwdm/scylla,eklitzke/scylla,scylladb/scylla,rluta/scylla,respu/scylla,acbellini/scylla,rentongzhang/scylla,stamhe/scylla,raphaelsc/scylla,glommer/scylla,eklitzke/scylla,gwicke/scylla,avikivity/scylla,eklitzke/scylla,acbellini/scylla,bowlofstew/scylla,kangkot/scylla,linearregression/scylla,scylladb/scylla,aruanruan/scylla,duarten/scylla,kjniemi/scylla,duarten/scylla,justintung/scylla,wildinto/scylla,asias/scylla,scylladb/scylla,avikivity/scylla,capturePointer/scylla,stamhe/scylla,rentongzhang/scylla,linearregression/scylla,avikivity/scylla,aruanruan/scylla,stamhe/scylla,bowlofstew/scylla,senseb/scylla,rluta/scylla,phonkee/scylla,tempbottle/scylla,senseb/scylla,rluta/scylla,wildinto/scylla,victorbriz/scylla,dwdm/scylla,phonkee/scylla,guiquanz/scylla,dwdm/scylla,respu/scylla,glommer/scylla,scylladb/scylla,shaunstanislaus/scylla,rentongzhang/scylla,victorbriz/scylla,kangkot/scylla,asias/scylla,shaunstanislaus/scylla,guiquanz/scylla,gwicke/scylla,tempbottle/scylla,kjniemi/scylla,capturePointer/scylla,capturePointer/scylla,wildinto/scylla,kjniemi/scylla