Would fftw_destroy_plan free the cache in fftw and slow down the creation of plan?

Here is the MWE:

```julia
using FFTW

x = rand(10000_0000)

GC.gc(true); GC.gc(true);

@time p0 = plan_rfft(x); # 3.775467 seconds (3 allocations: 224 bytes)
@time p1 = plan_rfft(x); # 0.003808 seconds (3 allocations: 224 bytes)
@time p2 = plan_rfft(x); # 0.004187 seconds (3 allocations: 224 bytes)

p0, p1, p2 = nothing, nothing, nothing
GC.gc(true); GC.gc(true);

@time plan_rfft(x); # 3.769389 seconds (3 allocations: 224 bytes)
```

version:

```
FFTW: 1.10.0
```