Query Templates: Adding dedicated /_search/template endpoint

In order to simplify query template execution an own endpoint has been added

Closes #5353
Recovery API

Adds a new API endpoint at /_recovery as well as to the Java API. The
recovery API allows one to see the recovery status of all shards in the
cluster. It will report on percent complete, recovery type, and which
files are copied.

Closes #4637
Add new / missing version constants
Add simple escape method for special characters to template query

The default mustache engine was using HTML escaping which breaks queries
if used with JSON etc. This commit adds escaping for:

```
\b  Backspace (ascii code 08)
\f  Form feed (ascii code 0C)
\n  New line
\r  Carriage return
\t  Tab
\v  Vertical tab
\"  Double quote
\\  Backslash
```

Closes #5473