* Implement basic "tsh db exec"
* adding ut
* minor refactor, fix race, rename iter func
* add help
* always use service name
* overwrite max connections with env var
* single get databases call
* remove prefix output
* fix some flags
* iterutils
* ensure each database
* add summery
* refactor, tests
* revert auto rename change by editor
* revert migrate
* remove unused var
* review comments
* renaming --max-connections to --parallel
* make exec return result instead of error
* hint TELEPORT_PARALLEL_JOBS
* fix golint
* address PR comments
