addrConn/ClientConn cleanup

[menghanl]

List of issues to be fixed:

• Fix the behavior of addrConn with empty address list Fix the behavior of addrConn with empty address list #1683

• return the underlying network error from ClientConn clientconn fails to return the underlying network error due to a recent change #1633

• Finer-grained locks or atomics for addrConn Finer-grained locks or atomics for addrConn #1669

• Invoke hangs in client after server presents an incorrect certificate Invoke hangs in client after server presents an incorrect certificate #1656

• Not send address's Metadata as metadata for RPCs on the connection (set here, and used here).
  Should do this explicitly through options (NewSubConn option?) instead.

• Keep retrying with exponential backoff even if the error is non-temporary.

[jeanbza]

Keep retrying with exponential backoff even if the error is non-temporary.

What's the rationale here? In the case of missing certs, this could be painful.

edit: referencing #1768

[menghanl]

@jadekler The reason is, other client behaviors depend on the fact that retrying always happens, for example, resolver re-resolving (#1795 (comment))