Add benchmark for VPIC-style workload

[ChristopherHogan]

Adds changes required for getting the best performance from write-only workloads.

• Control the stdio adapter's page size with the HERMES_PAGE_SIZE environment variable.
• Add rr_retry to api::Context. This is will retry failed round robin placements with the next Target. It mitigates the problem of several ranks doing a Put where each rank gets a valid schema, but only some of the ranks succeed in getting the buffers. For cases like VPIC where we know we have enough buffering space, we can enable this option to keep trying until GetBuffers succeeds.
• Added api::Context::disable_swap. Useful in tandom with rr_retry.
• RoundRobinPlacement now takes a split parameter to explicitly enable or disable splitting instead of leaving to random chance. This can be controlled globally via the default_rr_split config variable, or via the api::Context::rr_split option.
• Made several API arguments const.
• Removed RPC versions of ShutdownBufferOrganizer. The BO cannot be shutdown via RPC because the RPC servers need to be shut down before the BO (so that there are no outstanding operations in flight).

[coveralls]

Coverage decreased (-0.1%) to 81.19% when pulling 4be7aab on chogan/vpic2 into e10e815 on master.