* all: freezer style syncing

core, eth, les, light: clean up freezer relative APIs

core, eth, les, trie, ethdb, light: clean a bit

core, eth, les, light: add unit tests

core, light: rewrite setHead function

core, eth: fix downloader unit tests

core: add receipt chain insertion test

core: use constant instead of hardcoding table name

core: fix rollback

core: fix setHead

core/rawdb: remove canonical block first and then iterate side chain

core/rawdb, ethdb: add hasAncient interface

eth/downloader: calculate ancient limit via cht first

core, eth, ethdb: lots of fixes

* eth/downloader: print ancient disable log only for fast sync

* core, eth, trie: bloom filter for trie node dedup during fast sync

* eth/downloader, trie: address review comments

* core, ethdb, trie: restart fast-sync bloom construction now and again

* eth/downloader: initialize fast sync bloom on startup

* eth: reenable eth/62 until we properly remove it

* trie: fix merkle proof

* trie: use hasher instead of allocate keccack256 every time

* trie: add comments

This PR is a more advanced form of the dirty-to-clean cacher (#18995),
where we reuse previous database write batches as datasets to uncache,
saving a dirty-trie-iteration and a dirty-trie-rlp-reencoding per block.

* trie: disable fnv64a hashing of hashes for bigcache

* trie/database: add very important period

* cmd, core, eth, trie: get rid of trie cache generations

* core, trie: get rid of remainder of cache gen boilerplate

trie/database: fix overflow in parent tracking

* trie: return hasher to pool

* trie: minor code formatting fix

The current trie memory database/cache that we do pruning on stores
trie nodes as binary rlp encoded blobs, and also stores the node
relationships/references for GC purposes. However, most of the trie
nodes (everything apart from a value node) is in essence just a
collection of references.

This PR switches out the RLP encoded trie blobs with the
collapsed-but-not-serialized trie nodes. This permits most of the
references to be recovered from within the node data structure,
avoiding the need to track them a second time (expensive memory wise).

optimization code

* trie: reduce hasher allocations

name    old time/op    new time/op    delta
Hash-8    4.05µs ±12%    3.56µs ± 9%  -12.13%  (p=0.000 n=20+19)

name    old alloc/op   new alloc/op   delta
Hash-8    1.30kB ± 0%    0.66kB ± 0%  -49.15%  (p=0.000 n=20+20)

name    old allocs/op  new allocs/op  delta
Hash-8      11.0 ± 0%       8.0 ± 0%  -27.27%  (p=0.000 n=20+20)

* trie: bump initial buffer cap in hasher

* core, eth, trie: streaming GC for the trie cache

* trie: track memcache statistics

This removes a golint warning: type name will be used as trie.TrieSync by
other packages, and that stutters; consider calling this Sync.

In hexToKeybytes len(hex) is even and (even+1)/2 == even/2, remove the +1.

* all: get rid of error when create mdb

* core: clean up variables definition

* all: inline mdb definition