fix-1429 maxPoolSize and maxThreadSize should be configurable

[draganpodvezanec-verox]

closes #1429

The seeder thread pool configuration now uses environment variables with fallback to the original default values:

Core pool size: ${GWC_SEEDER_CORE_POOL_SIZE:16}
Uses the GWC_SEEDER_CORE_POOL_SIZE environment variable, defaulting to 16 if not set

Maximum pool size: ${GWC_SEEDER_MAX_POOL_SIZE:32}
Uses the GWC_SEEDER_MAX_POOL_SIZE environment variable, defaulting to 32 if not set

[groldan]

@draganpodvezanec-verox please rebase on top of main now that the build is working back