Operation: Use slightly more clever code to deserialize body

[Geod24]

Instead of copy-pasting blocks of code inside a `switch`,
we can just have a function instantiating the type and
greatly reduce duplication.

As `Body` is defined to be an interface to value instead
of an interface to pointer, a bit of magic is needed
with the `reflect` module to make it work.

[codecov-io]

Codecov Report

Merging #899 into master will increase coverage by 0.06%. The diff coverage is 47.36%.

@@            Coverage Diff             @@
##           master     #899      +/-   ##
==========================================
+ Coverage   60.28%   60.34%   +0.06%     
==========================================
  Files         153      153              
  Lines       10768    10749      -19     
==========================================
- Hits         6491     6486       -5     
+ Misses       3532     3518      -14     
  Partials      745      745

Flag	Coverage Δ
#integration_tests_long_term	44.44% <47.36%> (+0.05%)	:arrow_up:
#integration_tests_node	40.41% <47.36%> (-0.02%)	:arrow_down:
#unittests	48.7% <31.57%> (+0.09%)	:arrow_up:

Impacted Files	Coverage Δ
lib/transaction/operation/operation.go	43.33% <47.36%> (+4.8%)	:arrow_up:
lib/network/validator_connection_manager.go	84.18% <0%> (-0.8%)	:arrow_down:

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[Geod24]

@spikeekips : For this usage there will really have no impact, because we're essentially dereferencing a pointer. We are already depending on reflect very heavily through json and rlp encoding, to name a few things, who are doing real work with reflect (like looking up struct's fields). Also, as obvious in #836, it allows to reuse the same code for JSON and RLP decoding instead of copy/pasting this function.