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

* should fix import / export issues related to DB unavailability

* document reason for makeConfigNode

* fix comment

* comment consistency

* remove comments

* lint

* eth65 tests

linted

* remove non-latest eth light tests

This adds a lock around requests because some routers can't handle
concurrent requests. Requests are also rate-limited.
 
The Map function request a new mapping exactly when the map timeout
occurs instead of 5 minutes earlier. This should prevent duplicate mappings.

Fixes #21403

* core: avoid modification of accountSet cache in tx_pool

when runReorg, we may copy the dirtyAccounts' accountSet cache to promoteAddrs
in which accounts will be promoted, however, if we have reset request at the
same time, we may reuse promoteAddrs and modify the cache content which is
against the original intention of accountSet cache. So, we need to make a new
slice here to avoid modify accountSet cache.

* core: fix flatten condition + comment
Co-authored-by: Felix Lange <fjl@twurst.com>

Co-authored-by: Felix Lange <fjl@twurst.com>