storage_service: fix compilation on gcc 4.9
storage_service: fix compilation on gcc 4.9

gcc 4.9 fails to compiles the following legal code (which compiles fine on
gcc 5.1):

	#include <unordered_set>
	std::unordered_set<int> hi() {
        	return {};
	}

Work around this problem to make Scylla compile on gcc 4.9 again.

Note that this bug is specific to std::unordered_set - we have other places
in the code code which "return {}" for std::set, and those work fine.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@cloudius-systems.com>

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