Require either a root snapshot file, or Dynamic Startup flags

[jordanschalm]

The current Dynamic Startup behaviour is based on precedence:

• if the state is bootstrapped, we ignore the root snapshot file and Dynamic Startup flags
• else if a root snapshot file is provided, we use that and ignore Dynamic Startup flags
• else we use Dynamic Startup flags

Since Dynamic Startup is now more frequently used by existing operators, who already have a root snapshot file (from the spork), this arrangement leads to operators attempting to use Dynamic Startup, but inadvertently bootstrapping from an old root snapshot file instead.

This PR:

• Requires that strictly one of either a root snapshot file or Dynamic Startup flags are provided. If both are provided, we will exit and force the operator to indicate which bootstapping path they would like to use.
• Logs the root block ID and height after a successful Dynamic Startup
• Removes unused snapshot utilities (since merging the Protocol KVStore branch, a snapshot for any block can be used as a root snapshot).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 55.60%. Comparing base (c4d25c7) to head (a0e67e7). Report is 12 commits behind head on master.

Files	Patch %	Lines
cmd/dynamic_startup.go	0.00%	11 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5938      +/-   ##
==========================================
- Coverage   55.61%   55.60%   -0.02%     
==========================================
  Files        1128     1128              
  Lines       88928    88947      +19     
==========================================
- Hits        49458    49455       -3     
- Misses      34758    34778      +20     
- Partials     4712     4714       +2     

Flag	Coverage Δ
unittests	55.60% <0.00%> (-0.02%)	:arrow_down:

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