Remove servenv.exportBinaryVersion.

servenv.exportBinaryVersion md5sums the binary it is in at startup. This
meaningfully impacts startup time for all vitess binaries, since we need
to load several MiB of otherwise unused data from spinning disk.

This is not the appropriate way to manage binary versions. Attaching
this version metadata at build-time with the linker is much better for this.
An example of this is github.com/weingart/vendor, which we should
consider using.
Burn values into binary at build-time.

User, host, timestamp, and git-rev are values that we should capture at
build time to be able to track our instances across the fleet.
Use build metadata to fixup wrangler version checking.
Make wrangler version determination pluggable.

I'd like to use other mechanisms for this internally.
Merge pull request #229 from plobsing/exportBuildTags

manage build metadata