Mappings: Make lookup structures immutable.

This commit makes the lookup structures that are used for mappings immutable.
When changes are required, a new instance is created while the current instance
is left unmodified. This is done efficiently thanks to a hash table
implementation based on a array hash trie, see
org.elasticsearch.common.collect.CopyOnWriteHashMap.

ManyMappingsBenchmark returns indexing times that are similar to the ones that
can be observed in current master.

Ultimately, I would like to see if we can make mappings completely immutable as
well and updated atomically. This is not trivial however, eg. because of dynamic
mappings. So here is a first baby step that should help move towards that
direction.

Close #7486
DiscoveryWithServiceDisruptions: some more java docs and todos
release tool: must decode bytes to string
Discovery: only accept unicast pings when started

Due to component start order we may process an incoming ping while the ZenDiscovery module is not yet started. This leads to exception (from which we recover correctly, but the logs are note nice). UnicastZenPing should only start processing pings if it is started. We previously processed if not closed or stopped.

Closes #7950
Discovery: add a finalize round to multicast pinging

When sending a multicast ping, there is no way to determine how long it will take before all nodes will respond. Currently we send two pings (one at start, one after half timeout) and wait until the ping timeout has passed for all responses to come back. However, if all nodes are fast to respond, there is a gap relatively large between the moment that pings were gathered and the election that is based on them. This commits adds a last ping round (at timeout) where we know the number of nodes we expect to receive answers from. Once all nodes responded, we complete the pinging.

Closes #7924