sqlite3: Enabling option to disable filesystem locking (POSIX advisory locks)

When sqlite database is expected to be stored on a NFS disk (e.g., a
home directory and ~/.ndnx/ndnsec-public-info.db is on NFS mount),
default POSIX advisory filesystem locks may not work properly, since
some NFS implementation do not support them.  This commit allows
specification of ``--without-sqlite-locking`` to use alternative
"dotfile" sqlite locking mechanism.

Change-Id: Ieea69d2dff2ec61f3b5440b62f88f4e54dfda1c0
build: Experimental support to build using precompiled headers

This type of build needs to be explicitly enabled using --with-pch
configure option.

Change-Id: I95ba143c91fbb3f829a807b0f54d31ab343e4c3f
ci: Enable use of precompiled headers by travis-ci

Change-Id: I5221afcc967beeb7baf0a2f68c3fbffff80eaf97