Block builder interrupt, incremental block building option

[garyschulte]

Description

As a validator, I want besu to produce a block within a specified duration so that I do not miss block proposals.

In some cases besu might take a too long to build a full block, and when the consensus client asks for the prepared block, only an empty block is available. Conversely, if besu builds a block quickly (with few or low value transactions) and susequent high value transaction arrive after the block is constructed, besu will produce a suboptimal block.

Both of these situations can be mitigated by having besu's block building process continue to iterate on building the highest value block until it reaches a timeout threshold and/or it is interrupted via engine_getPayload.

Acceptance Criteria

• implement an interruptible block builder that will save a partial block on interruption
• implement continuous block transaction selection, to re-pack or re-build new blocks as new transactions arrive
• engine_getPayload should interrupt block creation, causing the current partial block to be saved (and if it is the highest value block yet produced, returned to the calling consensus layer)

[garyschulte]

relates to #4400