Multi-tier allocator patch

Part 2.
----------------------------
This patch introduces tryEvictToNextMemoryTier
and some additional multi-tier tests.

We can consider merging tryEvictToNextMemoryTier
with the initial implementation and seperating
the tests into a seperate patch.

Per tier pool stats
(multi-tier patch part 3.)
--------------------
This introduces per tier stats
this can go with multi-tier patch part 2.

Fix token creation and stats (#79)
(multi-tier patch 4.)
---------------------------------
This patch can go after we implement tryEvictToNextMemoryTier (or multi-tier part 2)
and should be combined as such.

* Fix issue with token creation

* Do not increment evictFail* stats if evictFailConcurrentFill were incremented

correct handling for expired items in eviction (#86)
(multi-tier patch 5.)
-----------------------------------------------------
This can be merged with patches that fix token creation
and probably squashed into multi-tier patch 2.

- we first check if an item is expired under mmContainer
  lock and if so mark it for eviction so it is recycled
  back up to allocateInternalTier.

cachelib/allocator/Cache.cpp

@@ -244,6 +244,7 @@ void CacheBase::updateGlobalCacheStats(const std::string& statPrefix) const {
       statPrefix + "cache.size.configured",
       memStats.configuredRamCacheSize + memStats.nvmCacheSize);
 
+  //TODO: add specific per-tier counters
   const auto stats = getGlobalCacheStats();
 
   // Eviction Stats
@@ -253,7 +254,8 @@ void CacheBase::updateGlobalCacheStats(const std::string& statPrefix) const {
   //   from both ram and nvm, this is counted as a single eviction from cache.
   // Ram Evictions: item evicted from ram but it can be inserted into nvm
   const std::string ramEvictionKey = statPrefix + "ram.evictions";
-  counters_.updateDelta(ramEvictionKey, stats.numEvictions);
+  counters_.updateDelta(ramEvictionKey,
+                        std::accumulate(stats.numEvictions.begin(), stats.numEvictions.end(), 0));
   // Nvm Evictions: item evicted from nvm but it can be still in ram
   const std::string nvmEvictionKey = statPrefix + "nvm.evictions";
   counters_.updateDelta(nvmEvictionKey, stats.numNvmEvictions);
@@ -295,11 +297,11 @@ void CacheBase::updateGlobalCacheStats(const std::string& statPrefix) const {
   }
 
   counters_.updateDelta(statPrefix + "cache.alloc_attempts",
-                        stats.allocAttempts);
+                        std::accumulate(stats.allocAttempts.begin(), stats.allocAttempts.end(),0));
   counters_.updateDelta(statPrefix + "cache.eviction_attempts",
-                        stats.evictionAttempts);
+                        std::accumulate(stats.evictionAttempts.begin(),stats.evictionAttempts.end(),0));
   counters_.updateDelta(statPrefix + "cache.alloc_failures",
-                        stats.allocFailures);
+                        std::accumulate(stats.allocFailures.begin(),stats.allocFailures.end(),0));
   counters_.updateDelta(statPrefix + "cache.invalid_allocs",
                         stats.invalidAllocs);