Increase APT::Cache-HashTableSize default to 50503

This drop the hash table utilization from a high 98%
to acceptable 74% on unstable, and the average bucket length
from 4.6 to 1.8.

This improves performance by about 5%, while increasing
the size of the cache by 0.2 out of 38MB, that is 0.5%.

48481 is a nice number

apt-pkg/pkgcache.cc

@@ -94,7 +94,7 @@ pkgCache::Header::Header()
    VerSysName = 0;
    Architecture = 0;
    SetArchitectures(0);
-   SetHashTableSize(_config->FindI("APT::Cache-HashTableSize", 15013));
+   SetHashTableSize(_config->FindI("APT::Cache-HashTableSize", 50503));
    memset(Pools,0,sizeof(Pools));
 
    CacheFileSize = 0;