rbd-target-api: concurrent requests are not supported

Hi everyone,

When two or more clients request rbd-target-api at the same time, the configuration file gateway.conf will be modified randomly.
When used at scale, performance is slow, will the community support concurrency?

Start by tracing the rbd-target-api request handling and the code that writes gateway.conf. Reproduce simultaneous requests to establish how the configuration is modified, then add coverage for concurrent access. Done means concurrent clients no longer cause random gateway.conf changes and the existing API behavior remains intact.

Written by the indexing model from the issue text.