Expose single obj causal

[cw75]

1. Added support for causal lattices (single obj). When two values have concurrent vector clock, the merge logic is to take a set union on the values.
2. I'm postponing cross-obj causal for now since we're having trouble finding its applications, but at least single obj causal is useful (it gives the client an opportunity to resolve conflict, instead of simply ignoring/overwriting a value like what LWW does).
3. Added a couple of simple tests for GET and PUT of causal lattices.
4. A few optimizations using std::move.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@afb4452). Click here to learn what that means. The diff coverage is 97.6%.

@@            Coverage Diff            @@
##             master      #68   +/-   ##
=========================================
  Coverage          ?   69.45%           
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  Files             ?       49           
  Lines             ?     1709           
  Branches          ?        0           
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  Hits              ?     1187           
  Misses            ?      522           
  Partials          ?        0

Impacted Files	Coverage Δ
kvs/include/kvs/lww_pair_lattice.hpp	100% <ø> (ø)
kvs/tests/kvs/run_server_handler_tests.cpp	100% <ø> (ø)
kvs/tests/kvs/server_handler_base.hpp	100% <100%> (ø)
kvs/include/lattices/core_lattices.hpp	95.83% <100%> (ø)
kvs/include/common.hpp	86% <100%> (ø)
kvs/tests/kvs/test_user_request_handler.hpp	100% <100%> (ø)
kvs/include/utils/server_utils.hpp	68% <83.33%> (ø)
kvs/include/kvs/vector_clock_pair_lattice.hpp	94.73% <94.73%> (ø)

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