Simplify API for accessing controller's head

[greg7mdp]

Just provide access to head's block_handle, and use block_handle member functions to access:

• block_num()
• timestamp()
• id()
• etc...

This is just a removal of non-necessary access functions, there is no semantic change.

See discussion with @arhag .

block_handle::internal() was made private in previous PR.

[greg7mdp]

I have updated the PR to also provide block_handle controller::fork_db_head() and deprecate other fork_db_head_* APIs.

[ericpassmore]

Note:start
group: CLEANCODE
category: INTERNALS
summary: Simplify API for accessing controller's head.
Note: end