Add spdk RPC for transient hold IOs in the spdk internal buffers

**Overview**
Implement a generic mechanism inside SPDK for temporarily holding incoming I/O requests in internal buffers.

Key Functionality
Transient Hold State: An RPC enables the hold state, causing SPDK to queue incoming bdev I/O operations in internal memory without rejecting them or forwarding them to the underlying storage tier.

Drain on Disable: Disabling the transient hold state via RPC immediately drains and dispatches all buffered I/Os sequentially to the backend.

Use Case
In Active/Active configurations, this RPC can be invoked during gateway failover to temporarily pause I/O traffic immediately before accepting a new blocklist epoch.

Primary Advantage
Provides a lightweight, low-overhead mechanism to pause I/O during brief state transitions, serving as an efficient alternative to transitioning the namespace to an INACCESSIBLE ANA state—which incurs significant overhead and triggers heavy host-side multipath re-pathing logic.

Start by tracing the existing RPC handling and the SPDK bdev I/O path described in the issue. Determine how transient hold state, internal buffering, sequential draining, and failover use would fit together. Done means an RPC-controlled hold and disable flow that buffers incoming I/Os without rejection and drains them to the backend.

Written by the indexing model from the issue text.