Update mercury and drop margo handles patch

See merge request zdvresearch_bsc/adafs!125

Since mercury fixed the issue on their code [1], we can drop the
temporary patch we were using to avoid handles reset.

Due to the mercury fix The follwing two issues have been solved:
 - Mercury: https://github.com/mercury-hpc/mercury/issues/272
 - Margo: https://xgitlab.cels.anl.gov/sds/margo/issues/47

[1]: https://github.com/mercury-hpc/mercury/commit/9512b8b04c265645f95fd99f3b9c015e836c0347

improved statfs and statvfs

See merge request zdvresearch_bsc/adafs!110

 - improved statfs system call
 - intercepted statvfs library call

Both of the above function now report the aggregated real size
of all the data-nodes.

You can test this with `df "/tmp/gekkofs_mountpoint"` command

mercury changed again its interface.
Before was `ofi+sockets://fi_sockaddr_in://<IP>:<port>` but now
it only works if we omit the intermediate protocol `ofi+sockets://<IP>:<port>`

They fixed a potential race condition:

https://github.com/mercury-hpc/mercury/commit/a421ea3072c79b2edb6d4d9d2bdaf179b5ef14f3

Margo fix handle cache

Closes #85

See merge request zdvresearch_bsc/adafs!116

See: https://xgitlab.cels.anl.gov/sds/margo/issues/47

Workaround must be reverted once fixed.

drop margo init patch

See merge request zdvresearch_bsc/adafs!115

bugfix: check empty string on has_trailing_slash

Closes #84

See merge request zdvresearch_bsc/adafs!113

Canonicalize input paths

Closes #81

See merge request zdvresearch_bsc/adafs!112

Changes have been merged upstream [1] with commit `acb8c296a694d7e3e06756e7a102a9d1c92df0c8`.
There is no more need to patch margo to expose enanched init function.

[1]: https://xgitlab.cels.anl.gov/sds/margo/merge_requests/9

since back() has undefined behaviour on empty string we need to ensure
that the string is not empty before to check the last char.

Fix access to non-existing map entry, code maintainence

Closes #83

See merge request zdvresearch_bsc/adafs!111

Paths passed as parameter to the daemon needs to be canonicalized

Remove optional dependency `gflags`

Closes #59

See merge request zdvresearch_bsc/adafs!109

`gflags` is an optional dependency of RocksDB, it is used only for cli
tools that we don't use.

Update mercury to V1.0.0

See merge request zdvresearch_bsc/adafs!108

Cleanup RPCs

See merge request zdvresearch_bsc/adafs!104

Since now there is only one margo instance there is no more difference
between RPC and IPC.

All the "ipc" occurence in the code have been removed in favor of "rpc"

removed `MARGO_FORWARD_TIMER` option that allows to measure RPCs timing