imap/xapian_wrap.cpp: C++ tweaks (#2810)

For iterators, prefer ++it over it++. The former creates a copy of
the iterator, modifies the original and returns the copy. The latter
modifies the original and returns it, without creating copies.

In the constructor of CyrusSearchStemmer pass "en" directly to the member
stem of type Xapian::Stem, which will call the Xapian::Stem constructor,
instead of creating a Xapian::Stem("en") object, and copying its
result in the member stem.

In xapian_snipgen_add_match: abolish variable r.

Change the type of make_lang_count_key to accept as second parameter
const std::string&. This avoids converting std::string to char* and
constructing from it a temporary std::string, instead of using all the
time the original string.

In xapian_snipgen_begin_doc: avoid calling two constructors for snippet.