Additional configuration options for FileChannelManager and, more generally, the pooling mechanisms

[hippiehunter]

Currently we open file channels and never close them for the duration of a FileChannelManager object. This improves the performance of individual requests, but it’s wasteful in situations where there are hundreds of files, but few that are commonly accessed. The plan for this is to implement a least-recently-used (LRU) eviction strategy. Keeping track of the total list of all files that have ever been opened and the relative frequency of file opens should allow us to auto-tune the number of channels to keep in the LRU list.