Speed up book close

[christopherlam]

speeds up my book destruction routines from 0.9s to 0.6s

here's benchmarking patch:

modified   libgnucash/engine/qofsession.cpp
@@ -127,6 +127,8 @@ QofSessionImpl::QofSessionImpl (QofBook* book) noexcept
 
 QofSessionImpl::~QofSessionImpl () noexcept
 {
+    float startTime = (float)clock()/CLOCKS_PER_SEC;
+
     ENTER ("sess=%p uri=%s", this, m_uri.c_str ());
     end ();
     destroy_backend ();
@@ -134,6 +136,11 @@ QofSessionImpl::~QofSessionImpl () noexcept
     qof_book_destroy (m_book);
     m_book = nullptr;
     LEAVE ("sess=%p", this);
+
+    float endTime = (float)clock()/CLOCKS_PER_SEC;
+
+    float timeElapsed = endTime - startTime;
+    PWARN ("quit=%f", timeElapsed);
 }
 
 void

and the result -- from UI, repeated reloading of my main datafile; before:

* 20:42:15  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.904586
* 20:42:20  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.885174
* 20:42:26  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.895748
* 20:42:35  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.887468
* 20:42:43  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.914600

after:

* 20:43:34  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.679304
* 20:43:40  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.645597
* 20:43:46  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.652250
* 20:43:51  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.691999
* 20:43:57  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.645178
* 20:44:02  WARN <qof.session> [QofSessionImpl::~QofSessionImpl()] quit=0.664965

[jralls]

OK.