Config option to change hard limit on term retrieval

[jamesamcl]

CC EVORA-project/ictv-ontology#16

[henrietteharmse]

We can look into increasing this limit may be to 10000. However, in load tests we have done last year fetching 100000 rows from 10 concurrent requests brought solr down. For that reason I will caution against implementing size=all.

[henrietteharmse]

A possible workaround is to make this a configurable parameter in the OLS backend. This will allow installations of OLS that could afford more hardware or installations with limited load, to change this and retrieve as many results as they want.

If such a config is added to OLS, the default behaviour (when no config is provided) should be to limit the number of results returned, with a large number of results returned only in the presence of a config.