double locks in gatt/server #14

In the gatt/server.rs file, there are potential double-lock issues that can occur in the following scenarios:

(1) A double-lock bug can happen in ServerCtx::execute_write() and ServerCtx::cccd_write()
The first lock is at L853, the second at L530.
Call Chain: execute_write()->do_write()->cccd_write()

(2) A double-lock bug can happen in ServerCtx::execute_write() and ServerCtx::csf_write()
The first lock is at L853, the second at L511.
Call Chain: execute_write()->do_write()->csf_write()