core/state/snapshot: reduce disk layer depth during generation

* eth/downloader, eth/handler: utilize sync bloom for getNodeData

* trie: handle if bloom is nil

* trie, downloader: check bloom nilness externally

eth/downloader: fix rollback issue on short chains

* metrics: zero temp variable in  updateMeter

Previously the temp variable was not updated properly after summing it to count.
This meant we had astronomically high metrics, now we zero out the temp whenever we
sum it onto the snapshot count

* metrics: move temp variable to be aligned, unit tests

Moves the temp variable in MeterSnapshot to be 64-bit aligned because of the atomic bug.
Adds a unit test, that catches the previous bug.

travis, dockerfile, appveyor, build: bump to Go 1.15

* core: define and test chain reparation cornercases

* core: write up a variety of set-head tests

* core, eth: unify chain rollbacks, handle all the cases

* core: make linter smile

* core: remove commented out legacy code

* core, eth/downloader: fix review comments

* core: revert a removed recovery mechanism

* t8ntool: add output basedir

* t8ntool: add txhash to trace filename

* t8ntool: don't default to '.' basedir, allow absolute paths

build: drop disco, enable groovy on Ubuntu PPAs

* cmd/geth/tests: try to fix spurious travis failure in les tests

* cmd/geth: les_test - remove extraneous option during boot

params: update CHTs for v1.9.19

ethstats: split read and write lock, otherwise they'll lock up

eth/downloader: allow all timers to exit

ethstats: overwrite old errors

eth/downloader: set deliverytime on drops and timeouts too

* les: close all connected les-server when shutdown

* les: linter nitpick

Co-authored-by: Martin Holst Swende <martin@swende.se>

eth/downloader: save the correct delivery time for state sync

les: update checktime even if check fails

* cmd/utils: grant snapshot cache to trie if disabled

* eth: fix up default non-mainnet cache distribution

* chaincmd should make config nodes instead of full nodes

* add documentation for using makeConfigNode instead of makeFullNode;

* add documentation to functions

* code style

core/vm: avoid map lookups for accessing jumpdest analysis