Cache samples requests, due to delays are repeated requests for Germinate

[Don-Isdale]

Context / Introduction

During testing of # 426 and # 427 in #423, timed-out responses were seen; this seems to be due to the number of samples rather than the density per se. It was requesting the samples multiple times - apparently re-requesting before the last request completes.

Proposed Solution

Caching the request promise should improve this performance.

[Don-Isdale]

Implementation complete

6980d7f Cache samples requests to improve Germinate performance

Testing shows this change performs as expected - there is a large reduction in the number of unnecessary requests.

Having removed the main source of redundancy, some other repeated requests are apparent :

• requests used to construct the VCF header for download from GUI; this is not required in the case of Germinate, and has been replaced in a change which will be in the following commit
• repeated click of the 'VCF Lookup' button before the request is complete.
  This is most noticeable with Germinate requests which are taking ~1min.
  Changes to solve this are in draft and will be in the next commit, described by Handle repeated click of the 'VCF Lookup' button before the request is complete #430.