Schema protobuf serialization part 2

[araddon]

Support schema serialization (protobuf) to enable distributed schema management. refs #199

Schema Serialization over the wire is a pre-requesite to a distributed schema Applyer

[codecov-io]

Codecov Report

Merging #206 into master will decrease coverage by 1.32%. The diff coverage is 33.99%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
- Coverage   68.12%   66.79%   -1.33%     
==========================================
  Files          87       87              
  Lines       16192    16435     +243     
==========================================
- Hits        11030    10977      -53     
- Misses       4310     4560     +250     
- Partials      852      898      +46

Impacted Files	Coverage Δ
datasource/json.go	40.62% <0%> (-0.43%)	:arrow_down:
datasource/membtree/btree.go	56.61% <0%> (-0.31%)	:arrow_down:
datasource/schemadb.go	59.5% <0%> (-9.76%)	:arrow_down:
datasource/introspect.go	36.45% <24.32%> (+0.09%)	:arrow_up:
datasource/files/storesource.go	67.12% <25%> (+0.45%)	:arrow_up:
schema/schema.go	46.71% <26.31%> (-42.5%)	:arrow_down:
datasource/files/filesource.go	61.53% <50%> (-0.37%)	:arrow_down:
datasource/sqlite/source.go	63.75% <50%> (-0.81%)	:arrow_down:
schema/registry.go	86.92% <60%> (+0.34%)	:arrow_up:
schema/apply_schema.go	67.93% <60.25%> (-32.07%)	:arrow_down:
... and 8 more

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