MINOR: connection: add a function to calculate elastic streams limit

MINOR: connection: add a function to calculate elastic streams limit

This adds a new tune.streams-elasticity parameter. This parameter
indicates, as a percentage, the average number of streams per connection
at full load. It is used to calculate limits of the number of streams to
advertise on new connections. 0 means that no such limit is set.

When a limit is set, the new function conn_calc_max_streams() determines
the optimal number of streams to allow on a connection. It will assign at
least the ratio of streams left to connections left, and at least a fair
share of what's left times the number of desired streams. It will always
ensure that each connection gets at least 1 stream, and everything beyond
this will be evenly distributed. For now the function is not used.