Rewrite #instrument

Only yields if block is given so #instrument can be called without one.

Clarifies conditional behavior when service is set.
Clarify method docs for #instrument
Include instrumentation_service with all new Connections
Document the #write, #read methods
Replace packet in write event payload with content_length
Rename response_id to object_type_id
Yield id, content_length even if indeterminate length

Turns out this could be useful even if it's unsupported.

Includes documentation.
Document instrumentation_service param for Net::LDAP.new