I2PTunnel: Reduce i2ptunnel threads, more thread pooling.

Big savings is on client side (two less threads per connection)
 - Move client pool from static inI2PTunnelClientBase to TCG.
 - Use client pool for some server threads
 - Run some things inline that were formerly threads
 - Client-side I2PTunnelRunner thread used to do nothing but start 2 more
   threads; now it runs one inline (like we do for server-side HTTP)
 - Javadocs and cleanups
Was originally intended to reduce load for high-traffic servers
but most of the savings for now is on the client side.
Ref: http://zzz.i2p/topics/1741
Todo: Figure out how to run the HTTP client-side gunzipper inline too
Todo: More server-side improvements

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Client side:

before:
4-5 threads, 1-2 pooled

  I2PTunnel Client Runner (BlockingRunner from client pool)
      starts I2PTunnelRunner or I2PTunnelHTTPClientRunner and exits
          starts StreamForwarder toI2P and waits
          starts StreamForwarder fromI2P and waits
              starts HTTPResponseOutputStream (HTTP gunzip only) (from client pool)
now:
2-3 threads, 1-2 pooled

  I2PTunnel Client Runner (BlockingRunner from client pool)
      runs I2PTunnelRunner or I2PTunnelHTTPClientRunner inline
          starts StreamForwarder toI2P and waits
          runs StreamForwarder fromI2P inline
              starts HTTPResponseOutputStream (HTTP gunzip only) (from client pool)

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Server side:

before:
1-4 threads, 0-1 pooled

  Server Handler Pool (Handler from server pool) execpt for standard server, blockingHandle() inline in acceptor
      starts I2PTunnelRunner or CompressedRequestor and exits
          starts StreamForwarder toI2P and waits   (inline for HTTP)
          starts StreamForwarder fromI2P and waits  (except not for HTTP GET)

now:
1-4 threads, 0-2 pooled

  Server Handler Pool (Handler from server pool) execpt for standard server, blockingHandle() inline in acceptor
      starts I2PTunnelRunner or CompressedRequestor and exits (using client pool)
          starts StreamForwarder toI2P and waits   (inline for HTTP)
          starts StreamForwarder fromI2P and waits  (except not for HTTP GET)