Speed up the LDAP Filter implementation

[tcalmant]

The LDAP Filter module (pelix/ldapfilter.py) is the slowest part of the code of iPOPO.
As it is often used, mainly for service filtering, it should be way faster.

According to line profiling, the evaluation of an LDAP filter takes more than 70% of the call time when using the get_service_references() method.