consensus/ethash: improve cache/dataset handling (#15864)

* consensus/ethash: add maxEpoch constant

* consensus/ethash: improve cache/dataset handling

There are two fixes in this commit:

Unmap the memory through a finalizer like the libethash wrapper did. The
release logic was incorrect and freed the memory while it was being
used, leading to crashes like in #14495 or #14943.

Track caches and datasets using simplelru instead of reinventing LRU
logic. This should make it easier to see whether it's correct.

* consensus/ethash: restore 'future item' logic in lru

* consensus/ethash: use mmap even in test mode

This makes it possible to shorten the time taken for TestCacheFileEvict.

* consensus/ethash: shuffle func calc*Size comments around

* consensus/ethash: ensure future cache/dataset is in the lru cache

* consensus/ethash: add issue link to the new test

* consensus/ethash: fix vet

* consensus/ethash: fix test

* consensus: tiny issue + nitpick fixes

consensus/ethash/algorithm.go

@@ -355,9 +355,11 @@ func hashimotoFull(dataset []uint32, hash []byte, nonce uint64) ([]byte, []byte)
 	return hashimoto(hash, nonce, uint64(len(dataset))*4, lookup)
 }
 
+const maxEpoch = 2048
+
 // datasetSizes is a lookup table for the ethash dataset size for the first 2048
 // epochs (i.e. 61440000 blocks).
-var datasetSizes = []uint64{
+var datasetSizes = [maxEpoch]uint64{
 	1073739904, 1082130304, 1090514816, 1098906752, 1107293056,
 	1115684224, 1124070016, 1132461952, 1140849536, 1149232768,
 	1157627776, 1166013824, 1174404736, 1182786944, 1191180416,
@@ -771,7 +773,7 @@ var datasetSizes = []uint64{
 
 // cacheSizes is a lookup table for the ethash verification cache size for the
 // first 2048 epochs (i.e. 61440000 blocks).
-var cacheSizes = []uint64{
+var cacheSizes = [maxEpoch]uint64{
 	16776896, 16907456, 17039296, 17170112, 17301056, 17432512, 17563072,
 	17693888, 17824192, 17955904, 18087488, 18218176, 18349504, 18481088,
 	18611392, 18742336, 18874304, 19004224, 19135936, 19267264, 19398208,