When a request that accepts text/plain is received, the threaddump
endpoint will now return a thread dump in plain text. The format of
this text is modelled after the output produced by JVisualVM when
connecting to a remote process over JMX. Note that this output does
not include all of the information in, for example, JStack's output
as it is not available via Java 8's ThreadInfo API.

Rather than the custom formatting logic, using ThreadInfo's toString()
method was considered but its output is documented as being undefined
and implementation specific. The implementation used while developing
this feature produced output that did not match that of JStack or
JVisualVM and truncated stack traces quite considerably.

At the time of writing the format produced by the endpoint could be
consumed by both Thread Dump Analyzer [1] and https://fastthread.io.

Closes gh-2339

[1] https://github.com/irockel/tda