Threadpool Info: Allow to serialize negative thread pool sizes

As a SizeValue is used for serializing the thread pool size, a negative number
resulted in throwing an exception when deserializing (using -ea an assertionerror
was thrown).

This fixes a check for changing the serialization logic, so that negative numbers are read correctly.

Closes #6325
Closes #5357
ok, so we need to remodel the `-1` value, which the user is allowed to supply in the configuration, see http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/modules-threadpool.html#_literal_fixed_literal