Avoid splitting log line across two files

Add a warning not to share consul nodes

driver/docker: pull image with digest

set killmode/killsignal in systemd example

Fix a typo

docs: fix typo in json documentation

GH #4290

Add digest support to the docker driver image config. This commit
factors out some common code to print the repo:tag (dockerImageRef) for
events/logs as well as parsing the image to retreive the repo,tag
(parseDockerImage) so that the results are consistent/sane for both
repo:tag and repo@sha256:... references.

When pulling an image with a digest, the tag is blank and the repo
contains the digest. See:
https://github.com/fsouza/go-dockerclient/blob/master/image_test.go#L471

Update minimum Go version.

In commit 758e1cb7 the `strings.Builder` API was used; this isn't available until Go 1.10+

Configurable TLS cipher suites and versions; disallow weak ciphers

Add job init '-short' command docs to website

Added deferred cancel to prevent context leaks

UI: Server-side reschedule tracking

We attempt to avoid splitting a log line between two files by detecting
if we are near the file size limit and scanning for new lines and only
flushing those.

BenchmarkRotator/1KB-8            300000              5613 ns/op
BenchmarkRotator/2KB-8            200000              8384 ns/op
BenchmarkRotator/4KB-8            100000             14604 ns/op
BenchmarkRotator/8KB-8             50000             25002 ns/op
BenchmarkRotator/16KB-8            30000             47572 ns/op
BenchmarkRotator/32KB-8            20000             92080 ns/op
BenchmarkRotator/64KB-8            10000            165883 ns/op
BenchmarkRotator/128KB-8            5000            294405 ns/op
BenchmarkRotator/256KB-8            2000            572374 ns/op

BenchmarkRotator/1KB-8            200000              5572 ns/op
BenchmarkRotator/2KB-8            200000              8338 ns/op
BenchmarkRotator/4KB-8            100000             14246 ns/op
BenchmarkRotator/8KB-8             50000             25279 ns/op
BenchmarkRotator/16KB-8            30000             48602 ns/op
BenchmarkRotator/32KB-8            20000             92159 ns/op
BenchmarkRotator/64KB-8            10000            154766 ns/op
BenchmarkRotator/128KB-8            5000            296872 ns/op
BenchmarkRotator/256KB-8            3000            551793 ns/op

Update serf to pick up graceful leave fix