Allow read-only threads on block producers

[nsjames]

There is an assertion that disallows setting read-only threads on a producer node.

spring/plugins/producer_plugin/producer_plugin.cpp

Line 1505 in f79d297

EOS_ASSERT(producer_plugin::test_mode_ || _ro_thread_pool_size == 0 || !is_configured_producer(), plugin_config_exception,

This seems heavy handed and could be a warning instead of a blocking error.

I've been running a local node to develop against for sync_calls and had to comment out that line so that I could run a stale producer that could also serve readonly API calls for unit tests.