Allow the --abi flag to be given - to indicate that it should read the
ABI information from standard input. It expects to read the solc output
with the --combined-json flag providing bin, abi, userdoc, devdoc, and
metadata, and works very similarly to the internal invocation of solc,
except it allows external invocation of solc.

This facilitates integration with more complex solc invocations, such
as invocations that require path remapping or --allow-paths tweaks.

Simple usage example:

    solc --combined-json bin,abi,userdoc,devdoc,metadata *.sol | abigen --abi -

* tracing: fix minor off-by-one error

* tracers: go generate

core: fix typo in comment code

cmd: cap cache size if exceeds reasonable range

rpc: set timeouts for http server, see #16859

* metrics: expvar support for ResettingTimer

* metrics: use integers for percentiles; remove Overall

* metrics: fix edge-case panic for index-out-of-range

* 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