Allow setting individual breakers to "noop" breakers

This adds a NoopCircuitBreaker, and then adds the settings
`indices.breaker.fielddata.type` and `indices.breaker.request.type`,
which can be set to "noop" in order to use a breaker that will never
break, and incurs no overhead during computation.

This also refactors the tests for the CircuitBreakerService to use
@Before and @After functions as well as adding settings in
ElasticsearchIntegrationTest to occasionally use NOOP breakers for all
tests.
[TESTS] Add a benchmark for circuit breaker overhead
Update plugins.asciidoc

Added Vietnamese Analyser to plugins page

Closes #6647
Eclipse: do not use star imports and reorganize imports on save.

Close #7587
Mappings: Store _timestamp by default.

Storing `_timestamp` by default means that under the default configuration, you
would have all the information you need in order to reindex into a different
index.

Close #8139
Search: Remove partial fields.

Partial fields have been deprecated since 1.0.0Beta1 in favor of _source
filtering. They will be removed in 2.0.