Remove non-public org.apache.accumulo.core.clientImpl.* usages

[SethSmucker]

fixes #3164 - .ClientConfConverter
fixes #3165 - .ClientContext
fixes #3166 - .ClientInfo
fixes #3167 - .ClientInfoImpl
fixes #3168 - .Credentials

part of #2443

At the moment, there are still some instances where TableLocator is the bottleneck for removing ClientInfo, ClientConfConverter and ClientContext, but this is only in two places. I've marked these with "//SETH NOTE" in the draft.

[SethSmucker]

Stacked PR Notice

This PR is stacked on #3253 and shares files with it.

PR	Title	Status
#3253	Remove interconnected IMA, Thrift, and ClientImpl	BASE - must merge first
#3345	Replace InMemoryInstance with InMemoryAccumulo	Also stacked on #3253
#3227	Remove non-public clientImpl.* usages	THIS PR (DRAFT)

Shared files:

• InMemoryAccumuloClient.java
• InMemoryClientInfo.java
• InMemoryConnector.java
• InMemoryInstance.java
• BulkInputFormat.java

IMPORTANT: Do NOT merge this until #3253 is merged. After #3253 merges, update this branch:

git checkout task/deps/3168
git merge origin/integration
git push

[SethSmucker]

This PR addresses tangled issues that must be fixed together.

Parent Issue: #3366 (Tangled: clientImpl APIs)

See #3366 for explanation of why these issues are interdependent.

[SethSmucker]

Superseded by #3253 and #3449 which cover all the clientImpl cleanup this PR was addressing.