[1.0.4] Fix fetch block with no block log

[heifner]

The startup of producer_plugin uses fetch_block_by_number to prime on_irreversible_block with LIB.

spring/plugins/producer_plugin/producer_plugin.cpp

Lines 1612 to 1613 in 838e885

	const auto lib_num = chain.last_irreversible_block_num();
	const auto lib = chain.fetch_block_by_number(lib_num);

When running without a block log, fetch_block_by_number always returned nullptr for LIB.

Update fetch_block_by_number and fetch_block_by_id to find LIB in fork database.

Resolves #1052

[ericpassmore]

Note:start
category: System Stability
component: Internal
summary: Fix fetch block with no block log. This removes unneeded warning by fixing underlying issue.
Note:end