Persistent asynchronous connections, credentials cache etc.

Hi Valery (and probably also Pavel @pavelbrylov),

we had tried to use the nginx-auth-ldap module in a quite high-performance application where I worked until recently and I made a few changes which might be interesting to you guys. Although not fully finished, my fork contains these improvements:
+ Uses asynchronous LDAP operations through nginx's event-driven framework
+ Creates configurable number of persistent connections to each server per each worker
+ Supports configurable cache per worker process for improved performance of consecutive requests
+ Transfers only the DN when searching, not the whole entry (several KB)
+ Allows only one LDAP (bind) operation per request when the whole user DN can be composed using variables
+ Has cleaner code and debug log messages
+ Contains other minor bug fixes

I am no longer able or willing to continue developing or maintaining the code because I have no use for it and I don't know how or whether at all will my successor at the company publish further additions or fixes, so I'll just leave it there (and here) and maybe someone will find what I did useful - the commits are separated nicely even for cherry picking.