Using an unconnected any_connection no longer crashes

[anarthal]

Refactored variant_stream and added tests

close #230 close #321

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.68%. Comparing base (b0c2639) to head (5bb3d1c).

Additional details and impacted files

[](https://app.codecov.io/gh/boostorg/mysql/pull/326?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ```diff @@ Coverage Diff @@ ## develop #326 +/- ## =========================================== + Coverage 98.51% 98.68% +0.17% =========================================== Files 138 138 Lines 7058 7069 +11 =========================================== + Hits 6953 6976 +23 + Misses 105 93 -12 ``` | [Files](https://app.codecov.io/gh/boostorg/mysql/pull/326?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) | Coverage Δ | | |---|---|---| | [...clude/boost/mysql/impl/internal/variant\_stream.hpp](https://app.codecov.io/gh/boostorg/mysql/pull/326?src=pr&el=tree&filepath=include%2Fboost%2Fmysql%2Fimpl%2Finternal%2Fvariant_stream.hpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-aW5jbHVkZS9ib29zdC9teXNxbC9pbXBsL2ludGVybmFsL3ZhcmlhbnRfc3RyZWFtLmhwcA==) | `100.00% <100.00%> (+9.82%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/boostorg/mysql/pull/326/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/boostorg/mysql/pull/326?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/boostorg/mysql/pull/326?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Last update [b0c2639...5bb3d1c](https://app.codecov.io/gh/boostorg/mysql/pull/326?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg).