hugolib: Add a cache to GetPage

hugolib: Add a cache to GetPage

Looks to be slightly slower with the low number of section pages, but the 1000 regular pages seem to add value.

```
benchmark                     old ns/op     new ns/op     delta
BenchmarkGetPage-4            97.7          145           +48.41%
BenchmarkGetPageRegular-4     7933          161           -97.97%

benchmark                     old allocs     new allocs     delta
BenchmarkGetPage-4            0              0              +0.00%
BenchmarkGetPageRegular-4     0              0              +0.00%

benchmark                     old bytes     new bytes     delta
BenchmarkGetPage-4            0             0             +0.00%
BenchmarkGetPageRegular-4     0             0             +0.00%
```