[C++] Do not hard code the "v" part of versions in thirdparty toolchain

[asfimport]

When I changed Flatbuffers from "v1.8.0" to a git hash, it broke the dependency download script. We should move all the version string to versions.txt rather than having some "v${FOO_URL}" in ThirdpartyToolchain.cmake

Reporter: Wes McKinney / @wesm
Assignee: Krisztian Szucs / @kszucs

PRs and other links:

• GitHub Pull Request #2587

_{Note: This issue was originally created as ARROW-3252. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2587
#2587