eth: only disable fast sync after success

eth/downloader: don't require state for ancestor lookups

* les, light: fix CHT trie retrievals

* les, light: minor polishes, test remote CHT retrievals

* les, light: deterministic nodeset rlp, bloombits test skeleton

* les: add an event emission to the les bloombits test

* les: drop dead tester code

The bulk of the issue was to adapt to the new requirement
that a v6 filter has to either contain a symmertric key or
an asymmetric one.

This commits revert one of the fixes that I made to remove
a linter warning: unexporting NewSentMessage. This is not
really a problem as I have a cleanup in the pipe that will
solve this issue.

* Remove --fast flag and clarify default

`--fast` is no longer a flag it's `--syncmode "fast"` and that is the default

* Remove --cache flag

--cache=512 is no longer required as of 1.8 as the default has been increased

* README: Minor cache amount fix, mention Rinkeby

* whisper: bloom filter refactoring

* whisper: fixed full node

* p2p/discv5: add query delay, fix node address update logic, retry refresh if empty

* p2p/discv5: remove unnecessary ping before topic query

* p2p/discv5: do not filter local address from topicNodes

* p2p/discv5: remove canQuery()

* p2p/discv5: gofmt

* params: update ropsten bootnodes

* params: fix linter

eth, light: minor light client startup cleanups

whisper: improve a log message to analyze a travis issue

This commit reduces database I/O by not writing every state trie to disk.

github: Replaces Wiki link [ci skip]

* les: limit number of LES connections

* eth, cmd/utils: light vs max peer configuration logic

cmd/utils: fix #16006 by not lowering OS ulimit

params: fix bootnodes gofmt

core, eth, les, light: get rid of redundant methods

ethdb: reset the batch size too on reset

* leveldb: Update leveldb to 211f780 (poolfix)

* core, ethdb: reuse database batches

* whisper: message format changed

* whisper: tests fixed

* whisper: style fixes

* whisper: fixed names, fixed failing tests

* whisper: fix merge issue in #15870

Occured while using the github online merge tool. Lesson learned.

* whisper: fix a gofmt error for #15870

* ethapi: add personal.signTransaction

* ethapi: refactor to minimize duplicate code

* ethapi: make nonce,gas,gasPrice obligatory in signTransaction

* whisper: fixes warnings from the code linter

* whisper: more non-API-breaking changes

The remaining lint errors are because of auto-generated
files and one is because an exported function has a non-
exported return type. Changing this would break the API,
and will be part of another commit for easier reversal.

* whisper: un-export NewSentMessage to please the linter

This is an API change, which is why it's in its own commit.
This change was initiated after the linter complained that
the returned type wasn't exported. I chose to un-export
the function instead of exporting the type, because that
type is an implementation detail that I would like to
change in the near future to make the code more
readable and with an increased coverage.

* whisper: update gencodec output after upgrading it to new lint standards

* whisper: fix empty topic

* whisper: add check to matchSingleTopic

* whisper: add tests

* whisper: fix gosimple

* whisper: added lastTopicByte const