Limit file handles used via remote_segments open

[jcsp]

Goal is to ensure that even if SI cache is large and client does a very large number of random reads that prompt segments to be opened, we impose a bound on the total number of file handles used by the process.

[jcsp]

Related: #6112

That PR adds a per-partition limit. Not as good as a global limit, but an improvement on the previous situation of having no limit at all + hitting bad_alloc.

[jcsp]

This is covered by #7042 -- we are mainly limiting the reader count for memory, but it'll help file handles too.