build: deprecate zesty, add bionic PPA

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