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

* trie: track memcache statistics

Use Read instead of Sum to avoid internal allocations and
copying the state.

name                      old time/op  new time/op  delta
CacheGeneration-8          764ms ± 1%   579ms ± 1%  -24.22%  (p=0.000 n=20+17)
SmallDatasetGeneration-8  75.2ms ±12%  60.6ms ±10%  -19.37%  (p=0.000 n=20+20)
HashimotoLight-8          1.58ms ±11%  1.55ms ± 8%     ~     (p=0.322 n=20+19)
HashimotoFullSmall-8      4.90µs ± 1%  4.88µs ± 1%   -0.31%  (p=0.013 n=19+18)

core: fix transaction event asynchronicity

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.

core/vm: fix typo in comment

This commit adds many comments and removes unused code.
It also removes the EmptyHash function, which had some uses
but was silly.

Fixes #16756

Fix a spelling mistake in comment

trie: cleaner logic, one less func call

* core: use a wrapped `map` and `sync.RWMutex` for `TxPool.all` to remove contention in `TxPool.Get`.

* core: Remove redundant `txLookup.Find` and improve comments on txLookup methods.

trie: support proof generation from the iterator

* vendor: update leveldb package

* ethdb: print warning log if db is performing compaction

* ethdb: update annotation and log

* node: all golint warnings fixed

* node: rm per peter

* node: rm per peter

eth: propagate blocks and transactions async

Fix some typos in comment code and output log

all: collate new transaction events together

travis, appveyor: bump Go release to 1.10.2