Improve connection pool configuration with explicit defaults

[dkropachev]

Summary

• Replace the CONNECTION_POOL_DEFAULT = 0 ("use SDK default") convention with explicit, Alternator-optimized defaults: maxConnections=400, connectionMaxIdleTimeMs=600000 (10 min), connectionTimeToLiveMs=0 (unlimited), connectionAcquisitionTimeoutMs=10000 (10 sec), connectionTimeoutMs=15000 (15 sec)
• Add two new connection pool settings: connectionAcquisitionTimeoutMs (controls wait time when pool is exhausted) and connectionTimeoutMs (controls TCP handshake timeout)
• Handle CRT HTTP client limitations gracefully: log info messages when zero-duration timeouts fall back to SDK defaults, log warnings when unsupported connectionTimeToLiveMs is set
• Apache sync client now explicitly disables the idle connection reaper when connectionMaxIdleTimeMs=0
• Log a warning when idle connection eviction is explicitly disabled (connectionMaxIdleTimeMs=0)
• Document all connection pool settings with defaults table in README

Breaking changes

• CONNECTION_POOL_DEFAULT constant removed (was 0)
• hasCustomConnectionPoolSettings() method removed
• maxConnections=0 now throws IllegalArgumentException (must be positive)
• Default behavior changes: pool settings are now always applied with explicit values instead of deferring to SDK defaults

Test plan

• Unit tests for new config fields, defaults, and validation
• Connection pool exhaustion tests for all HTTP clients (Apache, CRT sync/async, Netty) using real HTTP server
• Zero-timeout fallback tests for CRT sync/async
• Idle connection reaper enable/disable tests for Apache
• Connection TTL tests for Netty
• Warning/info log verification tests for CRT and config