[4.0 -> 5.0] Signal accepted_block after it is marked valid

[heifner]

Backport of #1386 hence the small diff.

As indicated in #1275, produced blocks were signaled to plugins after being marked valid and placed in the forkdb but signaled before being marked valid and placed in the forkdb for validated blocks. This PR makes sure that the block is marked valid before being signaled for both produced blocks and validated blocks. Also refactors location of updating head for applied blocks, consolidating it in commit_block.

This also fixes #1694 as head is updated consistently before accepted_block is signaled which is what state_history_plugin SHiP is using for reporting head in get_status_result_v0 messages.

Merges release/4.0 into release/5.0 including #1769

Resolves #1275
Resolves #1694