Remove a level of indirection when using CellInfo

The CellInfo struct contains a mutex, which interacts poorly
with some standard library containers which may move things around.
As a result we were using std::unique_ptr<CellInfo> in these
containers, but this change gets rid of that level of indirection
as std::unordered_map can construct CellInfo in place and we can
replace the use of std::vector with an array we know will not be
resized.

This improves the performance of the schur eliminator a bit but
also the performance of the block diagonal preconditioners.

Change-Id: If3ccd1273a754d9c5112e6e611ce31066b6b27b5