pack-priv final Class Http2Pool

This class is intended to be used only as HTTP/2 connection pool. It doesn't have generic purpose.

The connection is removed from the pool when:

• The connection is closed.
• The connection has reached its life time and there are no active streams.
• The connection has reached its idle time and there are no active streams.
• When the client is in one of the two modes: 1) H2 and HTTP/1.1 or 2) H2C and HTTP/1.1, and the negotiated protocol is HTTP/1.1.

The connection is filtered out when:

• The connection has reached its life time and there are active streams. In this case, the connection stays in the pool, but it is not used. Once there are no active streams, the connection is removed from the pool.
• The connection has reached its max active streams configuration. In this case, the connection stays in the pool, but it is not used. Once the number of the active streams is below max active streams configuration, the connection can be used again.

This pool always invalidate the PooledRef, there is no release functionality.

• PoolMetrics#acquiredSize(), PoolMetrics#allocatedSize() and PoolMetrics#idleSize() always return the number of the cached connections.
• Http2Pool#activeStreams() always return the active streams from all connections currently in the pool.

If minimum connections is specified, the cached connections with active streams will be kept at that minimum (can be the best effort). However, if the cached connections have reached max concurrent streams, then new connections will be allocated up to the maximum connections limit.

Configurations that are not applicable

• PoolConfig#destroyHandler() - the destroy handler cannot be used as the destruction is more complex.
• PoolConfig#evictionPredicate() - the eviction predicate cannot be used as more complex checks have to be done. Also the pool uses filtering for the connections (a connection might not be able to be used but is required to stay in the pool).
• PoolConfig#metricsRecorder() - no pool instrumentation.
• PoolConfig#releaseHandler() - release functionality works as invalidate.
• PoolConfig#reuseIdleResourcesInLruOrder() - FIFO is used when checking the connections.
• FIFO is used when obtaining the pending borrowers
• Warm up functionality is not supported

This class is based on https://github.com/reactor/reactor-pool/blob/v0.2.7/src/main/java/reactor/pool/SimpleDequePool.java