RemotePlugin: use atomic operation for lock

Use GCC's builtin atomic add/subtract operation for incrementing/
decrementing the recursive lock variable. This is needed to avoid race
conditions and is much faster than using mutexes etc.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f