reindex improvements

[David Bremner]

With the upgrade path for the body: prefix being "notmuch reindex
'*'", it's important that that is usable. It turns out that due to a
blunder on my part it wasn't really unless your whole mail store fits
comfortably in RAM.

[PATCH 1/2] CLI/reindex: fix memory leak

       This is an outright bugfix. 
       
[PATCH 2/2] n_m_remove_indexed_terms: reduce number of Xapian API

       This is optional, but the performance payoff is impressive. I
       suspect it might be even more impressive on spinning rust, but
       I didn't test it.