Add SplitKey server integration tests

[PeterHamilton]

This change adds integration tests that test registering, retrieving, and destroying SplitKey objects with the server. Minor updates are included for the client and server to ensure that SplitKey operations function as expected.

Partially implements #545

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@a8713fc). Click here to learn what that means.
The diff coverage is 40%.

@@            Coverage Diff            @@
##             master     #586   +/-   ##
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  Coverage          ?   96.28%           
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  Files             ?       75           
  Lines             ?    15808           
  Branches          ?        0           
=========================================
  Hits              ?    15221           
  Misses            ?      587           
  Partials          ?        0

Impacted Files	Coverage Δ
kmip/core/factories/secrets.py	82.14% <20%> (ø)
kmip/services/server/engine.py	99.23% <50%> (ø)
kmip/pie/factory.py	98.33% <66.66%> (ø)

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