Deprecate transaction manager creation methods that don't specify user agents

[nziebart]

Currently some overloads of TransactionManagers#create do not require a user agent, and thus use the default ("unknown-atlasdb"). There's no reason users should use these, so we should deprecate/discourage/delete these methods

[tpetracca]

So state as of #2459:

• all create methods are deprecated
• the builder optionally allows for a few ways to specify the user agent, which were chosen explicitly because they matched the existing power of the api of the various create methods. I'm more than happy to just break that immediately and make it mandatory to directly specify the user agent when you're using the builder (given that it's brand new and thus not really a break; I guess we've released since then, but shrug)

[hsaraogi]

User agent should be required when creating the TM.