Decoding a manifest from the JSON provided by C to the hash required by Perl is an expensive process. If manifest() was called on a remote it was being decoded into a hash and then immediately re-encoded into JSON for transmission over the protocol layer.
Instead, provide a function for the remote to get the raw JSON which can be transmitted as is and decoded in the calling process instead.
This makes remote manifest calls as fast as they were before 2.16, but local calls must still pay the decoding penalty and are therefore slower. This will continue to be true until the Perl storage interface is retired at the end of the C migration.
Note that for reasonable numbers of tables there is no detectable difference. The case in question involved 250K tables with a 10 minute decode time (which was being doubled) on a fast workstation.
Maintaining the storage layer/drivers in two languages is burdensome. Since the integration tests require the Perl storage layer/drivers we'll need them even after the core code is migrated to C. Create an interface layer so the Perl code can be removed and new storage drivers/features introduced without adding Perl equivalents.
The goal is to move the integration tests to C so this interface will eventually be removed. That being the case, the interface was designed for maximum compatibility to ease the transition. The result looks a bit hacky but we'll improve it as needed until it can be retired.
