__prev_message() -> _prev_message_is()

public -> private:
o head_message
o last_message
o next_message
o prev_message

prefix change:
o rfc822_message_ -> whole_message_
o get_first_plaintext_message() -> find_first_plaintext_message()
o get_encoding_mechanism() -> encoding_mechanism()
o get_data_type() -> data_type()
o get_data_type_list() -> data_type_list()

renamed:
o get_offset() - >offset_pair()
o header_in_body_part() -> message_fields()
o data_in_body_part()   -> message_text()
where we use RFC822 flabour naming convention.
rfc822 says "message = fields + text" and rfc2047 says "message" as
defined in RFC 822, with all header fields optional.

delayed loading by eval q{ use .. }.
validate defined( $db->{ _thread_list }->{ $id } ).

_head_thread(): fix head_thread search algorithm.

rename __search_default_next_thread_id() to
__search_default_next_id_in_thread().

__sort_yyyymm(): validate value is not null.