Improve performance for extremely small I/O workloads

[lukemartinlogan]

Hermes performs well at 4KB, but not so well at 300 bytes -- which is the case for certain workload types we are encountering.

1. Make a lock-free hashmap supporting both inserts and deletes
2. Improve shared memory allocator performance by leveraging lock-free queues for a small cache
3. Allow clients to read data directly from the SHM hashmap