* accounts/abi/bind/backends: Disallow timeshift for non-empty blocks

* accounts/abi/bind/backends: added tests for adjust time

* accounts/abi/bind/simulated: added comments, fixed test for AdjustTime

* accounts/abi/bind/backends: updated comment

params: update CHTs for v1.9.20 release

core/rawdb: only complain loudly if truncating many items

This change improves discovery behavior in small networks. Very small
networks would often fail to bootstrap because all member nodes were
dropping table content due to findnode failure. The check is now changed
to avoid dropping nodes on findnode failure when their bucket is almost
empty. It also relaxes the liveness check requirement for FINDNODE/v4
response nodes, returning unverified nodes as results when there aren't
any verified nodes yet.

The "findnode failed" log now reports whether the node was dropped
instead of the number of results. The value of the "results" was
always zero by definition.
Co-authored-by: Felix Lange <fjl@twurst.com>

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