mu-server: try avoiding xapian multi-threaded access

Try to avoid multi-threaded operations with Xapian.

This remove the thread workers during indexing, and avoids the indexing
background thread. So, mu4e has to wait once again during indexing.

We can improve upon that, but first we need to know if it avoids the
problem of issue #2756.

meson_options.txt

@@ -14,36 +14,56 @@
 ## along with this program; if not, write to the Free Software Foundation,
 ## Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 
-option('tests',
+
+option('cld2',
        type : 'feature',
        value: 'auto',
-       description: 'build unit tests')
+       description: 'Add support for language-detection through cld2')
+
+#
+# emacs
+#
+
+option('emacs',
+       type: 'string',
+       value: 'emacs',
+       description: 'name/path of the emacs executable (for byte-compilation)')
+
+option('lispdir',
+       type: 'string',
+       description: 'path under which to install emacs-lisp files')
+
+
+#
+# guile
+#
 
 option('guile',
        type : 'feature',
        value: 'auto',
        description: 'build the guile scripting support (requires guile-3.x)')
 
-option('cld2',
-       type : 'feature',
-       value: 'auto',
-       description: 'Compact Language Detector2')
-
 # by default, this uses guile_dep.get_variable(pkgconfig: 'extensiondir')
 option('guile-extension-dir',
        type: 'string',
        description: 'custom install path for the guile extension module')
 
+
+#
+# misc
+#
+
+option('tests',
+       type : 'feature',
+       value: 'auto',
+       description: 'build unit tests')
+
+option('xapian-single-threaded',
+       type : 'boolean',
+       value: true,
+       description: 'only use Xapian from a single thread')
+
 option('readline',
        type: 'feature',
        value: 'auto',
        description: 'enable readline support for the mu4e repl')
-
-option('emacs',
-       type: 'string',
-       value: 'emacs',
-       description: 'name/path of the emacs executable')
-
-option('lispdir',
-       type: 'string',
-       description: 'path under which to install emacs-lisp files')