Switch to BasicHttpClientConnectionManager instead of

PoolingHttpClientConnectionManager, and make the connection manager a member
variable of HttpRequest. We had been seeing a strange bug where once in
every few thousand urls or so, the connection manager would get gc'd
while the http request was being sent, and RecordingOutputStream.flush()
would get called, as in the stack trace below.  The ToeThread was using
it at the same time, so if conditions were right, the recorded stream
would get mangled. This commit makes the connection manager a member of
FetchHTTPRequest so it doesn't get gc'd prematurely. Also switches to
BasicHttpClientConnectionManager, since we're not doing any pooling.

  it.unimi.dsi.fastutil.io.FastBufferedOutputStream.dumpBuffer(boolean) line: 133
  it.unimi.dsi.fastutil.io.FastBufferedOutputStream.flush() line: 166
  org.archive.io.RecordingOutputStream.flush() line: 469
  org.apache.http.impl.io.SessionOutputBufferImpl.flushStream() line: 128
  org.apache.http.impl.io.SessionOutputBufferImpl.flush() line: 143
  org.archive.modules.fetcher.FetchHTTPRequest$RecordingHttpClientConnection(org.apache.http.impl.BHttpConnectionBase).close() line: 314
  org.archive.modules.fetcher.FetchHTTPRequest$RecordingHttpClientConnection.close() line: 520
  org.apache.http.impl.conn.CPoolEntry.closeConnection() line: 69
  org.apache.http.impl.conn.CPoolEntry.close() line: 95
  org.apache.http.impl.conn.CPool(org.apache.http.pool.AbstractConnPool<T,C,E>).shutdown() line: 130
  org.apache.http.impl.conn.PoolingHttpClientConnectionManager.shutdown() line: 347
  org.apache.http.impl.conn.PoolingHttpClientConnectionManager.finalize() line: 168
  java.lang.ref.Finalizer.invokeFinalizeMethod(java.lang.Object) line: not available [native method]