Serialize ops in account history objects

[theoreticalbts]

We need to serialize ops to get account history plugin to work with the new DB architecture:

• There's no way to propagate a custom allocator through static_variant
• Thus, static_variant is unusable from the database if any member types have fields that require dynamic memory allocation
• As a consequence, we can no longer store operation in any database object

The "fix" is to serialize history objects.

[theoreticalbts]

A fair amount of refactoring is needed to support this change.

Cleanup of where things are located:

• Move history objects to account_history_plugin
• Move impacted.hpp from app to account_history_plugin
• Update other plugins that reference impacted.hpp to require account_history_plugin at build time

Cleanup of notification API:

• Some plugins care about deserialized ops
• Therefore, object passed to pre/post apply hooks should be deserialized
• account_history_plugin serializes objects
• Therefore we need a new non-database type for storing unserialized operations. Call this type operation_notification

Since we're breaking the hook API anyway, I'll also modify it to have operation_notification be passed into both pre- and post-apply, instead of the current behavior (which is returning from pre-apply). This should avoid some unnecessary copying. For further savings, since it's no longer a DB object we can store a reference to the operation in the object.

[theoreticalbts]

The operation_object formerly played three different roles, which are now implemented by three different objects:

• steemit::chain::operation_notification : Used to send a signal to in-process plugin indicating that an operation has occurred. Operation is reference to transient object.
• steemit::chain::operation_object : Used to store operations in DB. Operation is serialized value.
• steemit::app::full_operation_object steemit::app::applied_operation : Used to return results in API. Operation is operation value.

[theoreticalbts]

There was a size check which didn't record operation history for operations of 128MB or less. This threshold seems unrealistically large, perhaps it was intended for 128KB? Regardless, I removed it.

[theoreticalbts]

Renamed full_operation_object to applied_operation to avoid check_reflect.py issues.

[theoreticalbts]

Merged.