Use absl containers for faster hashtable performance

We've had several instances where std hashtable performance matters: for rawtrace, where drcontainers was 15% faster: https://github.com/DynamoRIO/dynamorio/issues/2056#issuecomment-333167409, and https://github.com/DynamoRIO/dynamorio/issues/1738#issuecomment-809494133 (PR 4843). This issue covers importing absl containers for better hashtable performance (and other containers as well: probably targeting drcachesim analysis tools).