Remove pid-caching from BufferPoolAccessor

BufferPoolAccessor cached the pid via a static constructor. If the
process forks after this, then multiple processes generating unique ids
using the same pid value. This resulted in connection ID collisions.
use getpid(), which  already caches and resets appropriately across fork().

Bug: 142423602
Bug: 133186424
Test: boot, watch log connectionIds, collision-induced failures are gone