Portability - libc dependency

[jrolli]

Issue

The current configuration of the core library relies on sqlite3 which in turn relies on CGO and libc. This significantly reduces the portability of the Linux version of the released binaries. Specifically, the packaged version cannot be run on Ubuntu 18.04 because of a mismatch in the libc version.

Proposal

Change the sqlite3 dependency from using github.com/mattn/go-sqlite3 to modernc.org/sqlite. This should remove the libc runtime dependency and make the core library (and released binaries) significantly more portable across Linux distributions.

Concerns

• Dependencies on recent features of sqlite: This should not be a significant concern as the new dependency makes a concerted effort to stay up to date with upstream sqlite.
• Performance: The Go version of sqlite is not as performant as the CGO version in general. However, the database transactions are not generally on the critical path of cmgr functionality and shouldn't be a significant concern.

[jrolli]

As an additional advantage, this may make it easier to add experimental support for Windows containers/challenges. The last time I looked at it, there are some implicit POSIX assumptions but it mostly worked.